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Answers to Questions about Glass Types

By: Berlin Packaging Specialist
Date: January 26, 2020

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.

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