Glass Containers — Answers to Frequently Asked Questions

By: Berlin Packaging Specialist
Date: January 26, 2020

Glass Containers

Whether you currently use glass containers for your product packaging or are researching packaging options, you may have questions about the use of glass and its advantages and complexities. As the world’s only Hybrid Packaging Supplier, Berlin Packaging is the subject matter expert in glass containers. We are here to help answer your questions related to glass containers and materials.

If the following listed links do not answer your questions related to glass containers, please contact your packaging consultant for additional assistance.

Questions about Colored Glass

How is colored glass made?

The color comes from natural chemical elements which are added to the raw material mixture. Cobalt for blue, selenium for pink, graphite, and pyrite for dark yellow (amber), chromium for green.

How is glass color changed?

The color can be changed through a progressive but rapid change in the glass batch or by adding colorants. It takes a few days to clean the whole furnace from the previous color, since the molten glass mass amounts to about 100 – 300 tons, depending on the furnace surface. Moreover, glass has a slight elliptical movement and this causes the formation of pockets of resistance in corners of the furnace to the glass whose color is to be changed, which is also due to slight temperature variations in those positions and therefore different levels of viscosity.

It is also possible to color glass directly in the feeders, but in this case the feeders need to be specially designed so that they are the correct length to condition and homogenize the glass properly after the introduction of the “frit” (flakes of colorant material). In this way, it is possible to use clear glass on some machines and colored glass on other machines without having to change the contents of the whole furnace. In any case, this is a complicated and quite expensive procedure and only allows a limited range of colors.

What color offers the best protection from UV rays?

Antique green 99%, oak 98%, and amber 99%.

What does blue glass have that is not present in clear or green glass?

Basically, just a different use of colorants. For instance, cobalt is used in blue glass and chromite in green glass. Colorants provide filtration ability and protection from light, which is lowest in clear glass, slightly higher in blue, and more intense in the green.

Why do “antique green” bottles often show slight differences in color?

Because each glass factory uses different raw materials. At first, the “antique” color was patented both in the color and the name. Because of its great commercial success, many factories decided to launch similar products using different names and with slightly different raw materials. The weight of the bottle, or rather the thickness of the glass, can also lead to differences in color.

Why are bottles produced in different colors?

This is due to aesthetic reasons and for the protection of the contents, since some colors filter light.

Why are some glass containers dark in color?

The market requires different colors to protect different food products from light by using the filtering power of certain colors, and it is also for aesthetic reasons.

Why is dark glass used for red wine?

Aside from tradition, it is used to protect the product from light, since the wine is sometimes “aged” in these bottles. In fact, white wines, which generally have a shorter life, are often sold in clear glass bottles.

Why are wine bottles usually made of dark glass while tomato sauce jars are usually made of clear glass?

In the case of tomato sauce, the visibility of the product (mainly the color) is considered to be important for aesthetic reasons. Dark glass is often used for wine bottles in order to protect the product from light, but also for traditional and aesthetic reasons. Clear glass or light green bottles are also used to bottle wine.

Why do some bottles have a very shiny finish while others appear rougher and more opaque?

This is basically a result of the quality of molds and specifically: the material used for creating the molds; the maintenance of the molds; and the method used to lubricate the molds during production.

Why is blue glass exclusively produced in “color campaigns”?

This is due to the fact that the quantities of glass required by the market in this color, with the various different types of containers and the different geographical areas, cannot justify continuous production.

Why is glass not always completely transparent and sometimes "grey"?

This is a problem directly linked to the glass batch which is often made up of “cheap” silica sands or high quantities of cullet. Since the physical and chemical features of the final product do not change, this kind of glass is used for containers of widely used, everyday products where low cost is a primary requirement.

Why is oil generally packed in dark bottles in Italy and in clear glass bottles in other countries?

There is no clear rule about this. Perhaps Mediterranean culture requires a container that protects the product from light (because of the oxidation effect), especially in the case of high-quality olive oil. Nevertheless, many good quality products are also bottled with the so-called “mezzo bianco” or semi-clear glass (which is actually light green). This has a lower filtering power but allows better product visibility, and since the distribution system is very fast nowadays, the use of this glass does not create product problems.

There are some black or very dark glass bottles on the market. Is that a different kind of glass?

We generally call it black glass. Its composition is exactly the same as the glass in other containers but with a higher concentration of iron oxides, chrome, and manganese that practically block the passage of light. This material is frequently used for cream-based alcoholic drinks (mainly cream with milk and eggs).

Questions about Filling, Capping, & Finishing Glass Products

Should a glass container be washed before filling it for the first time?

When a glass container reaches the warehouse after production, it is clean because all the production phases guarantee this feature. Nevertheless, since the openings of the jars and bottles sometimes remain in contact with the packaging materials for quite a long time and condensation may form, the operator responsible for the final packing of the product before it is actually filled generally carries out an inspection using special equipment. The container is often turned upside down and blown with compressed air, or in other cases a precautionary wash is carried out with steam that is then powerfully aspirated in order not to leave any residue.

What is the “head space” in a bottle or jar? Why can’t jars be filled to the brim?

Independently from the filling method, head space is necessary to allow the expansion of the liquid contained, caused by variations in temperature. The percentage of head space compared to the volume of the container must be calculated according to the filling method and also according to the type of content. For example, alcoholic drinks require a head space of about 4–5% of the volume to allow for expansion of the product, while maple syrup — which is generally packed hot — requires a head space of 7% to ensure the nominal quantity after cooling. The filling level for bottles is indicated on the bottom, and it is expressed in the distance from the brim (in millimeters). The nominal capacity for jars indicates the milliliters of water that can be contained by filling them to the brim while the product content is generally expressed in grams. Jars that contain several different products (each product has different volume variations) require careful attention to the head space during the packaging process and the heat treatment which follows the filling and sealing operations.

Can glass containers be filled with hot products?

Yes, they can. For instance, fruit jams are poured into glass pots at about 85°C. The problem is the difference between the temperature of the glass and the temperature of the product.

Glass containers should be adequately “acclimatized”: it is recommended that they are not moved from the warehouses (that are generally unheated) directly to the filling plant. Careful attention should also be paid to the subsequent heat treatments when the containers are sealed (pasteurization and sterilization), since these treatments influence the internal pressure.

What is the significance of the “minimum through bore" and the internal profile of the neck?

These elements are of crucial importance because the glass container is filled using a steel tube. Failure to take account of the minimum through bore (internal diameter of the opening) can lead to breakage of the container or tube, damage to the filling process, failure to complete the filling phase, etc. The internal profile of the neck refers to the stoppering profile inside the neck down to below the finish, while the minimum through hole applies to the whole length of the bottle neck.

What is the difference between natural cork and synthetic cork?

With current technology, there are some fundamental differences between the two closure systems. A natural cork adapts better to the inside of the neck, allowing the appropriate, slow transmission of air, which is ideal for the natural aging of wine. It also has no expiry date, unlike synthetic corks whose already low elasticity gradually disappears with time when not in use. When a synthetic cork is used, particular attention is required during the bottling process because it is necessary to adopt an air pre-vacuum system while introducing the cork (in order to prevent the cork rising again) and the jaws should be perfectly closed. Natural cork, on the other hand, is much more flexible and even private users can insert them using a very simple machine with no other tools necessary. Obviously, using synthetic corks for short periods avoids the problem of cork taint. Moreover, synthetic corks do not release any particles into the liquid and they are therefore often used for spirits, in the form of the so-called “mushroom cork”.

How is the size of a synthetic cork determined?

The answer is simple for synthetic corks since there are only two standard sizes for diameter and two for length, that is to say ø 22 x h 38 or h 42 for still wines and ø 23 x h 38 or h 42 for sparkling wines. Unfortunately, at present, few manufacturers have a synthetic cork suitable for containing pressure from gases.

Do different types of closures on bottles affect the contents?

Closures for jars depend on the type of product contained and on the heat treatment used. Over time, the need for products that could last longer while retaining their principal features, thus extending their distribution, has encouraged the development of new sealing and packing systems.

Why do labels on bottles come off quite easily or have wrinkles?

The reason can range from incorrect application methods or a lack of compatibility between the label glue and the surface treatment of the container to the glass shape that may have only one radius of curvature in the label application area. It often depends also on the label material, whether it is paper, PVC, or something else, and on the ability of the material to adapt its shape on application to glass. All treatment processes can be carried out well or badly and labels also fall into this category of quality control issues, which include the features of the glass, the selection of the most appropriate material, the quality of the labeling system, the overall quality of the production process, and the ability of the worker to adjust all the components so that the work may be carried out appropriately.

What causes the “waves” that are visible around the lower circumference of a clear glass bottle?

These waves are left by the “parison” and caused by the different temperature of the blank mold at the points where the glass ends after the gob is loaded. Careful adjustment of the production equipment helps to prevent this optical effect (that is even more visible on clear glass bottles). Progressive pressure valves are used to limit this inconvenience. Once the bottle is filled, 90% of the optical effect disappears and the mechanical strength of the bottle is not affected. Other shadows on the body of the bottle may be caused by the incorrect loading of the glass gob.

Why does a white film sometimes appear on glasses and bottles?

The cause of this white film is the humidity that is present in many environments, which creates a thin film on the surface of the containers. This film tends to draw out alkalis (sodium and calcium) from the surface of the glass, increasingly over time, until the contact liquid has completely evaporated. The resulting cloudy look is caused by the formation of carbonates due to the presence of CO2 in the air. Usually, washing or filling the glass will remove the film. It is important to note that this is just an “aesthetic” defect that doesn’t affect the use of the container and doesn’t cause any damage to the product inside. If glass is unused for a long time and kept in damp environments (in holiday homes for example) or after prolonged use, this process of extraction can become almost irreversible, with the accumulation of large quantities of insoluble calcium carbonate. This may result in a cloudy, rough surface caused by micro-abrasions and insoluble calcium deposits — a defect that is visible but harmless in terms of health.

Are bottle caps suitable for all types of bottled products?

It depends on the product characteristics and on operational requirements in order to achieve the longest and best shelf life for the products. From the old methods of sealing with oil and natural corks, we have now arrived at a host of increasingly specialized and specific closure solutions to deal with all possible problems involving protecting the packed product from the external environment. These solutions also seek to offer the consumer ways to detect any chance or intentional interference with the original packaging (tamper-proof seals and flip caps, etc.).

How are screw caps manufactured and why are they categorized as pasteurizable or sterilizable lids?

Screw caps are manufactured from tinplate sheets: they are cut into disks which are then deformed at their edges using special chucks which form the traditional “skirt” and the threads that allow the cap to fasten onto the glass jar. In order to ensure a perfect coupling between the jar and the lid, some “soft materials” called mastics are used, which need to be resistant to heat treatments to enable the preservation of the contents of the jar. More specifically, pasteurization at a temperature below 90°C; sterilization between 120 – 125°C.

How is the height of a screw cap determined?

These parameters are fixed by international standards, which establish diameter and height along with other data, e.g., ø 28 top and bottom.

Are metal and plastic screw caps interchangeable?

Some kinds of caps have the same gauge for the screw thread and therefore they are interchangeable as long as there are no specific requirements deriving from the contents or the filling procedures.

What is the most suitable way to fit the caps and lids — manually or automatically?

Both methods are suitable provided that the entire packaging process is correct. The automatic method ensures a more consistent and uniform application of the closure to the glass container.

Why are there different tops for bottles?

The type of closure system used must be selected according to the contents of the bottle/jar, but the traditional mode of consumption is often the main factor influencing this decision. Closures themselves, whether made of cork or synthetic material, have no technical limitations in their use. Aluminum screw caps — very useful because they can be reclosed — are used mainly for carbonated drinks, spirits, or oil.

Why do some bottles on the market use non-refillable caps?

Non-refillable caps are required by law in accordance with art. 18 of Law 161/2014, which regulates the distribution chain for virgin olive oils to prevent false imitations. The same principle is applied to alcoholic drinks, for marketing purposes, although it is not currently required by law. In this case, the closure also serves to control the flow when pouring.

How can we be sure that supplemental decoration will dot “detach” from the bottle?

The term glass decoration means modifying the appearance of the container with a second processing. There are different ways to decorate glass. The most commonly used methods on bottles and jars are silk printing, sandblasting, acid frosting, and painting.

Can the decoration or application of a frosted finish on a bottle be harmful for its contents?

These operations are only harmful if the interior of the bottle mouth is not kept adequately protected during the procedure.

Are there any size limitations for glass containers in countries inside or outside the EU?

Containers are divided into two categories: measured containers (mainly bottles) and non-measured containers (pots/jars). The first category shows the capacity of the container on the bottom and the symbol 3 (reversed epsilon). The capacity is pre-determined, depending on the type of liquid contained.

For instance, in EU countries, alcoholic drinks can be in bottles of 700 ml or 1500 ml, while in the U.S, bottles can be 750 ml or 1750 ml. Other capacities are used in both geographic areas (250 ml, 375 ml, 500 ml, 1000 ml). Non-measured containers are generally pots or jars, where the volume of the container does not correspond to the volume of the content. These containers are produced to give the user the ideal volume for packaging a certain quantity of the product. In this case, the label provides the necessary information, stating the product weight, the amount of preserving liquid and the symbol “e”, which means that the filler is responsible for providing the content details. There are no general rules that apply to these containers because they differ according to market sector, national units of measurement, normal market practice etc.

Who decides the weight of a bottle and on what basis?

There are a number of factors that influence the weight of a bottle: the bottle volume; the minimum thickness required for the sides and neck of the bottle to make it possible to use; the need to provide specific axis loads or to ensure a certain resistance to internal pressure (e.g., champagne bottles); and finally aesthetic requirements.

Why do bottles have the same capacity but different glass weights?

This is due to different manufacturing requirements, for example the shape of a bottle may require a higher quantity of glass (e.g., for edges) or the use of the bottle may require a higher mechanical strength (for carbonated drinks or champagne), or for marketing purposes, because a heavier bottle appears to be more valuable or because the thickness of the glass can result in different shades of color.

Questions about Glass for Food & Drink

Why do some products only require pasteurization while others need sterilization?

The pasteurization process — which destroys microflora — must be immediately followed by rapid cooling of the product and it is suitable for acidic products (such as fruit juices, tomato sauce, beer, etc.).

The sterilization process is necessary for food products with low acidity (e.g., fish, meat, vegetables, etc.), which are more susceptible to bacterial infection that may create pathogens. The increase in temperature from 95°C for pasteurization to 125°C for sterilization enables the bacteria to be destroyed and, with the creation of a vacuum inside the container, the contents to be preserved. When the button at the center of the lid is depressed, it shows that the container is properly sealed and the vacuum inside the container has been maintained. When the button is raised, this shows that the vacuum in the container has been lost and the safe preservation of the contents can no longer be guaranteed.

Why is glass the most hygienic material for packaging purposes?

Because glass is completely neutral to its content: it neither releases anything nor absorbs anything from the product (taste, odor, or aroma). It has a high chemical resistance and is waterproof and gas-proof. It can be easily sterilized, has antistatic properties, and does not pollute the environment. It is completely indifferent to climate variations.

Why is there sometimes a numeric code on glass containers?

EC Law 178/2002 specifies traceability requirements for food products and the need to be able to retrace every single step of the food supply chain. EC regulation 2023/2006 governs GMP (Good Manufacturing Process) rules. This is why many glass factories — in order to comply with the above rules — print numeric codes on their bottles which indicate the production day, time, or other production details even once the container has been filled and separated from the production identification code attached to the pallet. Generally, this printing is carried out with special inks which are visible only under UV light or by using laser technology.

The use of traditional indelible ink is very rare. Filler companies — which are directly obliged to make their products totally traceable — print their reference codes on the glass or the lid using inks which are visible to the naked eye.

Why isn’t shatterproof glass used for food containers?

This is possible because it is a secondary process, but shatterproof glass is not generally used for large scale production of containers because it is unnecessary for the type of use and would be much more expensive because of the procedure.

What is the "plastic grid" used for?

The plastic grid, usually made of food-safe plastic, is used in jars to keep products entirely submerged in their preservation liquid (oil or other liquids).

What is the difference between cosmetic glass and glass for food products?

The difference lies essentially in the use of specific raw materials in the mixture, which tends to favor products that enhance “shine” (by using barium for example) rather than the mechanical features. In cosmetic packaging, nickel alloy cast iron is used for the molds in order to increase the glossy finish of glass containers.

Is there a special process for making beer bottles?

Bottles that are especially designed for classic or craft beers (with fermentation in the bottle) are not manufactured using particular technical features except for the resistance to pressure, since — like the champagne method — they must resist up to 6 bar. They are generally made in amber or dark green glass for better protection from UV rays.

What is the difference between an ordinary glass bottle and a bottle used for sparkling wine?

There are some technical differences: in sparkling wine bottles the shape of the bottle is different (e.g., bottles with sharp edges are not suitable), the weight of the glass needs to be of sufficient thickness to withstand the internal pressure, and there is a picure (punt) or a heavier, stronger base. Moreover, they also need to have a uniform distribution of the glass.

How is the size of a cork for a wine bottle determined?

The size of a cork (width) is always calculated taking into account the type of wine (still or sparkling) and the internal caliber of the container at a depth of around 40 – 50 mm (about 18.5 mm on average). For still wines, this diameter +6 mm is used for natural corks or +4 mm for technical cork stoppers such as the double-disc cork. Sparkling wines always have double-disc corks, so 4 mm are added.

For spumantes (Italian sparkling wine), a single kind of cork is used that is ø 30.5 mm (the cork starts off larger in order to maintain the pressure). The cork height is 40 mm due to problems in remaining air-tight and because it is the minimum size possible for the cutting and processing of cork. This can vary from 40 to 54 mm for various reasons but mainly for marketing purposes. A cork of 48 mm in height is always used for spumante.

Sometimes in wine shops there are very big champagne bottles. Are they as functional as ordinary bottles, or do they only have an aesthetic purpose?

They are certainly as functional as the ordinary bottles, but they are subject to very strict quality controls because of their size and their sparkling content. They are generally used in sizes from Magnums to Methuselahs, and more rarely Salmanazars and Nebuchadnezzars and their resistance is tested up to 16 bar. The last four are used very rarely because of dangers related to champagne pressure in such high quantities. They have been used for sparkling wines, as in the case of the 27 liter “primat” (which corresponds to 36 ordinary bottles) to celebrate Moser’s Hour record in Mexico City.

What is the "picure" or "punt" used for?

The picure or punt was originally used to collect wine sediment. Today, it is used to guarantee better resistance to pressure for sparkling wine or fizzy drinks or for aesthetic reasons linked to tradition.

Questions about Glass Breakage

At what temperature does glass "break"?

Glass bottles and jars are usually not affected by ambient, refrigeration, or warm temperatures. However, high heat (>300°F) and excessive thermal variations can cause glass to shatter or break. Glass is a poor thermal conductor and rapid changes in temperature (roughly 60°F and greater) may create stress fractures in the glass that may eventually crack. When heated, thin glass begins to crack and typically breaks at 302–392°F.

If a glass container is placed on a very hot source of heat (e.g., 500°C), it can gradually lose its shape and change from a permanent solid form to a plastic state.

In general, glass jars should not be heated in a microwave or oven. Glass jars made of ordinary glass may crack or explode in a microwave. Some glass containers are made of heat-resistant materials and can withstand microwaving. Consumers can find the “Microwave Safe” sign at the bottom of the glass products that are designed to be used in microwaves. However, secured lids should never be used when microwaving glass containers.

Glass may break when subjected to temperatures below freezing. This may occur because the contents freeze and their expansion cause the glass to crack (if the cap does not come off).

Why does hot water cause glass to break?

It is not the hot water that breaks the glass, but the sudden change in temperature, causing internal stress to be exerted on the material. If these changes occur suddenly, they create internal tension that leads to the breakage of the container. Glass is a bad heat conductor and therefore it does not tolerate excessive changes in temperature. Normally, it can tolerate about 45°C, so if, for example, the application requires the temperature to reach 90°C for pasteurization, it is necessary to gradually increase the temperature of the environment where the container is situated. This method will prevent any problems even with the sterilization process, which reaches 130°C. It is necessary to use an even more gradual procedure for containers of a particular shape (jars with handles, sharp edges, extra large containers, etc.).

Why do some bottles bounce off the ground without breaking after falling from a height, while others shatter?

Whether a glass container breaks or not depends not only on the type of impact but also on the thickness and distribution of the glass as well as the level of annealing.

What does “shatterproof glass” mean if the glass then breaks?

This is a “current” term, which is not completely accurate. It refers to glass that has undergone a particular process called “tempering”, which brings the object to a temperature of about 600°C and then cools it suddenly (with cold air distributed according to the thickness of the glass), causing controlled raised tensions inside the object. In practice, a layer of tensile stress is created, bounded by two layers of compressive stress, giving the object increased resistance. The molecular structure which is created allows the glass to shatter into very small particles in the event of a sharp impact, and these cannot cause damage.

Questions about Glass Recyclability & Cleaning

Who collects glass for recycling and who uses it?

Glass is collected by companies appointed by the individual municipalities. The collectors treat the glass cullet in special industrial plants that wash it, break it into very small pieces, and separate off the impurities. The recycled glass is then sold to glass factories that mix it in with the batch formula.

Is glass “biodegradable?” What impact does glass have on the environment?

No. Glass is a mineral and the labels on glass containers generally state “please dispose of responsibly”. Over time, the glass will tend to “return to sand”, especially if it is thrown away on beaches, in the sea, or wherever it may be repeatedly moved by natural elements.

How much recycled glass should be used in production?

There is no technically fixed quantity. Cullet can be used as a support to the melting of the raw materials since it melts at a lower temperature and thus offers significant economic and environmental advantages (less energy consumption). For extra-flint glass, in particular, the quantity of glass cullet is kept as low as possible (10%) in order to avoid impurities appearing in the recycled material.

Another solution is for the producer to use cullet exclusively from their own recycled product. Some glass factories that produce colored glass can use 60% or more of cullet since any color impurities in the cullet will not have a significant impact on the final result. It is also possible to manufacture new containers using only cullet, but in this case, it is important to adopt special strategies to keep complete control of the material in order to ensure the homogeneity of the molten materials.

Is it possible to recycle glass that is already a product of recycling?

Glass recycling can be repeated indefinitely; a 600 gram container will produce another container of the same weight without any loss. The original “mineral” does not change: the process of melting to the semi-liquid form is repeated (some people would define glass as a highly viscous liquid), the shape is changed, it solidifies as it cools, and is then used again.

Is it possible to detect glass packaging with recycled content?

No. When “cullet” is melted, it returns to its original purity as if it was “new glass”.

Since glass objects are thrown indiscriminately into glass banks regardless of their color, what color is the final recycled glass product?

A recycled glass of mixed colors is generally used to produce dark glass (green, antique green, or yellow). Cullet (crushed and mixed) is used in the normal mixture of raw materials, with the addition of colorants that guarantee a uniform final result. Melting takes place at over 1500°C and at this stage there is a viscous, perfectly homogeneous and purified mixture.

Why are glass water bottles returned and how are they reused?

This is a question of relative cost, balancing the cost of water with the cost of the glass bottle, as these are both “low cost” items. It is necessary to take into account the cost of returning the bottle to the filling plant, as well as the cost of the necessary processing before it can be reused: such processing is quite complex in terms of equipment and water consumption for washing in order to ensure the completely hygienic condition of the bottle independently from its previous use.

Why shouldn’t lids be reused?

Because the mastic used for the lid, the material which allows the “air-tight” seal, doesn’t have the same elasticity after the first use. The “threads/lugs” that ensure the coupling of the lid to the jar finish become deformed after the first use.

If glass containers should be “cleaned” before they are used by the food industry, what is the most efficient cleaning method?

Glass containers arrive at the end of the uninterrupted production chain clean and packaged. Before using the container, the filling company must ensure that no internal contamination has occurred during storage and unpacking. A washing or blowing procedure is generally adopted on the production line. The blowing procedure — which is the most commonly used — is carried out by a machine placed in between the depalletizer and the product filler, which blows air forcefully into the inverted container. In this way, the combination of air pressure and gravity together eliminate any impurities which may have been introduced during the storage or unpacking phases.

Questions about Glass Types

What is Type III glass? Are there other types of glass?

This is a classification of glass for containers which has been adopted by different Pharmacopoeias in order to establish a more appropriate use of glass in containers according to their contents. There is glass type I, type II, and type III.

Type I is a borosilicate glass (known as Neutral) with a high hydrolytic stability suitable for containing injectable products.

Type II derives from type III: thanks to a special ammonium sulfate treatment of the internal surface, this type achieves a similar hydrolytic stability to type I and is suitable for containing acid or neutral products (e.g., infusion solutions).

Type III is a soda-lime glass with a low alkali content and good hydrolytic stability, suitable for containing preparations that are not in aqueous or alkaline sensitive solutions.

There is another type of glass which is suitable for containing food products and known as Type A Glass (soda-lime glass) which does not require any specific hydrolytic resistance, unlike type III.

Are tumblers, household objects, windows, etc. all made of the same type of glass as pots, jars, and bottles?

There are different types of glass which are all manufactured by the same process of melting raw materials, based on silicon, soda, and potassium, in a furnace. These various types of glass meet different requirements. For instance:

  • Tableware, consisting of the traditional glass tumblers and containers. The batch used contains a higher percentage of BaO (barium oxide) — to give more shine and transparency — and a sand with a low percentage of iron oxides. We refer to this glass as “long glass”, meaning that it has a lower melting point.
  • Pirex: this type of glass must have a low expansion coefficient so that the container can resist sudden changes of temperature. The chemical composition of the batch is therefore different: it contains boron and is called “borosilicate glass”. The same type of glass is used in the amber-colored glass — with higher resistance to light — for laboratory or pharmaceutical containers.
  • Crystal: this is obtained by adding lead oxide (up to 35%), which gives it its shiny finish and the typical crystal sound.
  • Plate glass: apart from the different chemical composition of this type of glass — to give greater resistance — a whole chapter could be devoted to the production methods. 90% of the plate glass produced worldwide is manufactured using the “float glass process”: the molten batch is poured into a long molten tin bath in controlled atmosphere. The glass floats on the tin and spreads along the bath surface, thus creating smooth surfaces on both sides. The glass cools down and becomes solid as it flows along the bath, creating a floating ribbon. Then the product is flame polished so that the surfaces are perfectly parallel. This kind of glass is considered to be unsuitable for building purposes because it tends to break into large, sharp pieces. In order to remedy this problem, when plate glass is subject to impact or static stress, the individual plates have to be tempered by being heated in a furnace up to 600°C and then suddenly cooled down by forced drafts of cold air. Two or more glass plates are coupled in order to increase thermal insulation (for windows or glass walls) and they are kept separated by air or gas (argon, krypton, or xeno) or they are kept together by using plastic film according to their final use.

What are the main components of glass?

The main components of the glass batch are:

  • silica (sand with particular features), which is the glazing element;
  • soda, which is the melting element;
  • calcium carbonate, which is the stabilizer.

When was glass first used as an industrial packaging material?

The ancient Romans used glass to store their wine, but industrial production with mass volumes did not begin until the late 1800s.

What is the difference between ordinary glass and crystal?

Glass is a generic term that includes different types of the same material. Crystal is a particular type of glass principally made up of lead, barium, and zinc which are all substances that increase the refraction index responsible for its shine. The batch used to produce crystal always requires slightly lower melting temperatures than glass for bottles. Moreover, crystal is not as hard as traditional glass and therefore it can be more easily engraved.

What are the main features that differentiate special bottles from standard bottles?

There isn’t any real classification in this respect. A standard bottle is one that is usually manufactured by a glass factory and that can be used by many different customers. It is generally produced in large quantities compared to the average production of a glass factory.

Special bottles may refer to those with a particular shape, not square or round, but may also refer to those designed especially for a particular customer and sold exclusively to that customer.

From the point of view of the technological production, there is absolutely no difference between these two articles, but special bottles always require a particular expertise when the production starts, in printing brand names, dealing with irregularities in the shape or creating a heavy base, or because they are unstable on the conveyor belts.

Sometimes when a glass bottle or jar breaks you can see that the glass thickness is not uniform. Why is this?

The glass container is formed in the blow mold by a blow-blow process. The stretching of the glass is therefore influenced by the temperature of the glass batch. The reduction in glass thickness is often more evident in bottles and usually appears at two-thirds of the bottle height, going down towards the bottom, which corresponds to the height of the parison. So a certain amount of difference in the thickness is a natural physical feature, provided it does not affect the solidity of the container.

Many bottles have a very thick glass bottom. What is this for and how is it made?

The extra thick glass on the bottom of bottles is primarily for aesthetic reasons to make the bottle look more valuable, and in the case of clear glass, to enhance the transparency and purity of the bottle. Technically, the concentration of glass on the bottom of the bottles is achieved by creating a special preparatory mold (parison) with a higher glass mass than a normal bottle that does not get deformed during the first blowing process.

Questions about Glass Manufacturing

How long does it take to manufacture a bottle and at what temperature is the glass processed?

It takes 10 to 15 seconds to transform the glass gob into a bottle that is immediately sent to the annealing lehr in order to eliminate the surface tensions in a process that lasts 1 to 2 hours. Manufacturing glass is both an art and science.

How is the mold changed?

This is one of the most complex operations in a glass factory and it must be carried out by one or more teams of qualified technicians.

When the required quantity of an article has been reached, production is stopped by deviating the glass gobs from the feeders into special cooling water tanks instead of into the molds. Some technicians start by working on the feeder mechanisms to set the new glass weight, if necessary replacing the cuvettes through which the glass gob passes and setting the new parameters for the entire feeder pipe. Others work on the machine sections, replacing the molds and preparing the new production process, including the transfer of the new articles to the annealing lehr. These two phases are carried out at very high, uncomfortable temperatures, with the additional psychological pressure of the “time factor”, since factories work on a continuous production cycle.

The change of molds can take 2 to 6 hours, or even 1 full working day, depending on the production, to finish setting it up. While these operations are being carried out in the so-called “hot” area, another team is setting up the new parameters on electronically controlled machines in the “cold” area, where products are selected and packed, using — where possible — a range of defective samples of the new article which have been kept from previous productions.

Which parts of the molds are interchangeable and what changes can be made without having to remake the entire series of molds?

  • Finish equipment group (neck-ring, guide-ring, and plunger), provided that it fits the diameter of the neck.
  • The bottom, with or without a punt or to allow different capacities.
  • The blow mold allowing a neutral or customized version of the same bottle.

What is the difference between a single gob and a double gob?

A double gob is when two gobs are collected at the same time from a feeder using a cuvette with a double orifice.

In some cases it is possible to collect 3 or 4 gobs at the same time, that all go into the same forming section of the machine (obviously into different molds). The machine can have 6, 8, 10, or even more sections. With the use of more glass gobs, it is possible to produce more articles in the same production time unit.

Are bottles made by welding two halves together?

No. The vertical lines that are visible to a greater or lesser extent on bottles and jars correspond to the joining points of the two half-molds.

What is "sampling inspection?"

A production batch can be satisfactorily evaluated using a statistical control method (internationally regulated by a specific reference standard), which consists in the random sampling of a certain number of pieces in proportion to the overall quantity of the production batch. The results of this sampling inspection should be representative of the quality of the whole batch.

Why can a glass furnace never be turned off?

This is a technical feature of continuous production that is common to blast furnaces as well. The melting temperature of 1500°C takes around 12 days to reach, with very high energy consumption. Thus, the furnace cannot be turned on and off at will. Moreover, the glass material in its cooling phase could cause damage to the delicate parts of the furnace such as the “throat”, and the refractory material that the furnace is made of goes through some highly critical points during certain changes in the temperature range (approximately between 1200°C and 1100°C and between 900°C and 800°C).

Since it is the result of an industrial operation, can glass have internal impurities? If so, how can it be considered to be the most hygienic material?

It is important to distinguish between glass purity (the raw material melts at about 1500°C, while the shaping of the container takes place at about 900°C and these temperatures “purify” everything) and its overall hygienic quality, which depends firstly on how the container is packed at the glass factory and then on how it is stored and used later by the food packing company. Packaging processes — if properly carried out — normally ensure the maintenance of the same hygiene conditions that apply during the production process itself. So if we are talking about “glass” as a material, it is hygienic by definition, but if we are talking about glass containers (bottles or jars), the hygienic qualities depend on how the container is used.

Is it possible to create any kind of container with glass?

Theoretically, there are no size or shape limitations. It is generally the customer who decides, based on the production process (manual or industrial), or economic aspects, etc.

What is the reason for the stippling on the bottle base?

The stippling helps to keep the bottle stable on the conveyor belt during production and to keep the bottle punt (which is still at 650°C) separate from the metal belt, preventing cuts to the bottle base through contact with different temperatures.

What are the small dots on the bottom of a bottle for?

The sequence of space and dots represents a mold code number and allow electronic control of the production process and of that particular mold.

What are the engravings (letters and numbers) on bottles for?

They represent a sort of identity document for the bottle: they contain the name of the producer with its factory trademark, the brim capacity and the filling level, the number of mold cavities used to produce the bottle, and the symbol which indicates that the bottle/jar is suitable for food products.

  1. Nominal capacity in cl or ml
  2. Symbol for measuring container bottle*
  3. Filling level
  4. bottom with dots
  5. Symbol indicating it is suitable for food
  6. Manufacturer’s trademark
  7. Reference cavity number
  8. Bottom with crescents

* The reversed epsilon similar to a number “3” is the symbol established by EC regulations to identify measuring container bottles. This refers to containers which have the necessary metrological features guaranteed by the glass manufacturer, which make it possible to measure the actual contents with the appropriate accuracy when filled to a certain level from the brim (EC directive 106/75).

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