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Answers to Questions about Glass Types, Recycling, and Cleaning
By: Berlin Packaging Specialist
Date: January 26, 2020
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 glass 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 glass 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.
Answers to 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.
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.