Colour is the most obvious characteristic of glass. We perceive it before the shape and even before the cut design. It can also be one of the most interesting and beautiful properties of glass. Colour sometimes determines the usefulness of a glass object, where it protects the contents from UV rays, for example, but it almost always determines its attractiveness and desirability. Let's take a look at how glass was coloured at the dawn of history and how it is coloured today. At what stage in the glassmaking process is the colour added and what common and bizarre substances are used to colour glass.

At the dawn of history

Around 3500 BC, the first real glass was produced in Mesopotamia and ancient Egypt. Some of the first coloured glass objects were beads and small jars. These early glassmakers discovered that adding different substances to the melt would change the resulting colour. Once methods of making coloured glass were discovered, an explosion of experimentation began. The aim was to find substances that would produce specific colours in the glass.

In the eighth century, the Persian chemist Abu Musa Jabir ibn Hayyan, often known simply as 'Geber', recorded dozens of recipes for making glass of specific colours. Geber is often referred to as the "father of chemistry", due to his recognition that metal oxides were a key ingredient for colouring glass. (Read more about the history of glass here.)

Stained Glass

Among those who provided incentives to the first glass artisans were mainly religious organizations. Stained glass became a very popular addition to churches, mosques, synagogues and other important buildings more than 1,000 years ago. The artists who produced these artistic windows needed a variety of colors to create a realistic stained glass scene. Their search for a full palette of colors was the impetus for further research and experimentation that led to the creation of a vast array of stained glass.
 

How is glass coloured today?

Each glassworks has its own traditions and its own secret recipes for creating a range of colours. So don't expect exact recipes for specific shades, each glassmaker guards the exact recipes like an eye in the head. What we can reveal to you, however, is the dyeing process. And there are curiosities, such as the use of uranium oxide or the use of cow poop to dye glass. But more on that later.

Glass colouring process

The natural colour of the glass is greenish. It must be de-coloured. For this, sodium nitrate or manganese dioxide is used. Once the colourless mixture is ready - which is the standard - it is ready to start.

Mix the solids

Although the following description may seem simple, it is definitely not. Painting glass is a precise job, so it is important to measure, prepare and plan everything carefully. The metal oxides used to colour glass are stored in bulk form. These oxides are added directly to the bulk mixture of the glass strain and everything goes into the mixer. The mixture must be absolutely homogeneous, so no expense is spared in mixing.

Put it in the pan

The perfectly homogenous mixture is then loaded into a huge melting pan and melted until it forms the melt that is formed into the crystal products as we know them.

It must be accurate

Each time the colour is mixed in, the shade must be tweaked to match one hundred percent. With the finest glassworks, it is not possible for individual batches of coloured melt with the same name to vary by even half a shade. Therefore, each time the paint is mixed, a "control log" is first created to assess whether the paint is mixed correctly. Because if the paint is not mixed exactly right, the whole 350 kg batch goes to waste and that is a shame.

Mix the bulk, stir, melt.

Proven recipes

Glass colours are a constant, experts say it is possible to create any colour, but each colour has to go through chemical development and research before it can be mixed. This is time-consuming and by the time such development occurs, the colour may no longer be on trend. That's why glassblowers stick to tried and tested glass colours, recipes that are up to 160 years old and have been handed down through generations.

Glass like a chameleon

As a result, the colored glass is subjected to output control under different light variations. It looks completely different under artificial light and natural light. The thickness of the glass wall also plays a role. On a thicker wall, the colour looks more concentrated than on thin-walled glass. You also need to take into account which colour suits which cut, as not all glass colours are ideal for cutting. Colors also affect the hardness of the glass. In short, it's not easy to colour glass, it's all really a science. That's why it's a good idea to stick to proven recipes.

Colour palette

The recipe for making stained glass usually involves adding metal oxide to the glass, we already know that. The following table lists some of the metal compounds and the colours they produce. Manganese dioxide and sodium nitrate are also listed. These are decolourisers - materials that neutralise the colouring effect of impurities in glass. The table is to be taken as a guide only, because, as we have already said, the specific compounds and recipes are left to each smeltmaster.

Just to give you an idea, there are many more colours, as well as compounds that are used.

Don't confuse colored with sprayed

Although this is probably obvious at first glance, we prefer to point it out anyway. Not all coloured glass is made of coloured melt. There is plenty of glass that is tinted by spray painting after the product is made. For example, the entire popular Rainbow collection, or Plantica collection. With quality Czech manufacturers these colours are permanent, durable and completely safe, plus it looks really nice, what do you think?

The cherry on top

Atomic glass

Did you know there is glass that is coloured with uranium? Uranium oxide to be precise, often with iron oxide mixed in to enhance the greenish colour. You'll find it under names like Uranium Glass, Atomic Glass, Petroleum Glass. This glass was mainly produced between 1850 and 1920, when it was most popular. During the WWII all uranium was confiscated and uranium glass production ceased. It wasn't until 1959 that production picked up again, but not spectacularly, as uranium was and still is very expensive.

To this day there are absolute enthusiasts and collectors of Atomic Glass who have extensive collections. The glass is pale greenish lime yellow and when exposed to UV light, it has a beautiful, perhaps a little eerie glow. Strange as it may seem, this glass is not radioactive in any way, as uranium oxide is stable enough not to cause radiation. This glass is absolutely fascinating and definitely worth seeing.
 

left uranium glass on normal light / right on UV light

Cow poop glass

Our Czech ancestors knew how to make use of absolutely everything. The amber colour of glass that we still love today was created by our ancestors with an organic substance added to the melt. František N. Paraubek, the general director of the Czech-Moravian Glassworks, states in his book Glassmaking Discussions (1944) that the tried-and-tested ones include "...hay, straw, sawdust, soot, coal, graphite and cow dung. "* We cannot imagine how the glassmakers of that time figured it out; it could be a funny story. Anyway, we're so glad they threw that poop in the melt, because Amber is a gem and we'd love to drink from it anytime.

*Quote from the article "Glasses from cow dung, Museum of Wallachia"

Beautiful color Amber, unfortunately these pieces don't contain any cow poop :)

Remember that glass is insoluble in water, as are all its components, so without further ado, "Cheers".