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Answers to Questions about Colored Glass

By: Berlin Packaging Specialist
Date: January 26, 2020

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).

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