The Secrets of Colored Glass!!!

For esthetical reasons, colored glass is highly appreciated and there are many people who  collect colored glass items. Some do this from a historical point of view (Depression glass) and some just because they like the colors.

Colored glass tumblers (Princess Glass World)
Colored glass tumblers


The Ancient civilisations in Mesopotamia, Phoenicia and Egypt made glass by simply mixing the raw materials sand, lime, and soda ash. The resultant glass was opaque or semi-transparent. It also retained whatever color was imparted by natural impurities. In time the glass makers learned where the colors came from and were able to produce glass with an intense color. Through the use of different metals and metal oxides they were soon able to produce blue glass, turquoise, yellow and black glass and even opaque white glass. The glass was used mainly as parts of jewellery and in the time of the Ancient Egyptians the material was seen as so very special that it was not available for the common people but it was exclusively used for the Pharao and the people around the court.

Amberina crytal pitcher

The Middle Eastern craftsmen improved the quality over the years and around the start of the Roman Empire the glass makers were able to make semi-transparent clear glass and also semi-transparent colored glass. The Ancient Romans used this colored glass for holloware but most of the time they used to cut colored glass into a variety of shapes and then set those pieces into mosaics. 

The Byzantines further refined the glass making process and tried to reach full transparency but only in the 15th century the famous glass makers in Venice were able to make fully transparent glass. The called it ‘cristallo’ after the mountain crystals they found nearby in the Dolomite mountain range.

The English glass maker George Ravenscroft invented in 1675 the clear lead crystal by addition of lead oxide to glass (click here for more info about crystal).
Lead crystal became very popular for making glassware and the glass makers started to make colored led crystal.   
Cut crystal vase with
purple glazing

Amberina Glass, ruby-red glass
The most popular colored crystal became the ruby-red amberina. The color was like the color of the gemstone Ruby. 
The ancient Romans already knew that adding gold to glass would convert it into a ruby-red material when heated in a controlled fashion. But this knowledge combined with the invention of lead crystal by George Ravenscot, led to beautiful ruby-red crystal and all the famous glass makers in Europe, like Baccarat, started to make glassware out of this material.

In the US the glassware industry was during those days not as big as in Europe, so the European companies exported their glassware to the US.
In 1883 it was the company New England Glass Co., later named Libbey, who produced the first US amberina. Libbey was very keen to patent this Amberina recipe for the US and this gave the company a monopoly among the US glassmakers. Libbey produced amberina until about 1920. 

Today there are still companies around the world who produce glassware from this beautiful intense red crystal. The problem is that gold is expensive and numerous attempts have been made to replace the gold by another element. The most succesful alternative to gold was by using selenium in the selenide form. With selenium glass also can have a nice red to deep red color. However with selenium the process is not as reproducable as with gold.

Purple Glass 
Set of green cut crystal wine glasses

Manganese is probably the oldest applications of a colorant to glass as the old Egyptians already discovered it. manganese, also used as a discolorizing agent (see ‘Decolorization of Glass’ below), is used to make purple glass. Because manganese can be present in different oxidation states next to each other, one can see another effect in purple glass: a green and orange fluorescence. 

Green Glass
One of the most powerful coloring agents used in the glass making industry is chromium. Chromium is used in the production of dark green glass. If too much chromium is added, then the color approaches black.

Green Depression Glassware
(Uranium glass)

In the old days iron was used to make green glass. The iron color was more an olive green. The disadvantage of iron is that you need to add considerably more iron than chromium, in order to reach the same color intensity. Iron is sometimes used in combination with chromium.

A very special fluorescent green can be made with traces of Uranium. Everybody knows that uranium is radioactive but this type of glass has only little uranium content so it is not dangerous to the health as such. However the powdered glass, during breakage or during polishing, is carcinogenic.

Turquoise Glass 
Copper is another very powerful coloring agent and is nowadays used to make turquoise glass. Copper is a versatile material and it can also be used to make blue glass, green glass and even red glass. To make these other colors is a very complicated matter so now it is used to make turquoise glass only. 

Turqoise vase (Lalique)

Blue glass 
The oldest form of blue was probably made with copper during the time of the Ancient Egyptians. In the Roman Empire this glass was very popular and was called Egyptian Blue Glass.
A much easier way to make blue glass is the addition of cobalt. We find cobalt already present in 7th century glazes for pottery in the Middle East and in Europe it showed up during the last half of the Middle Ages in stained glass.
Cobalt gives a very nice deep blue color and is still very popular among glass makers and consumers. Libbey and Arc International use cobalt for their blue glassware.

Pink glass
When selenium, a non-metal, is added to glass the color of the glass becomes usually pink. 
Pink glassware has always been very popular. In the Depression Glass period it was used for many items but also nowadays pink is very popular especially for champagne flutes. 

Champaign flutes with pink crystal

Cobalt blue tumbler 

Amber glass
When iron is added to glass in combination with sulfur and carbon one obtains amber colored glass. The carbon itself has nothing to do with the final color and is only used as a reducing agent to bring the iron and the sulfur in their reactive state to form the amber color. When these ingredients are used in larger quantities one can make black glass. 
Another way to make amber glass is using silver in quite large quantities.

Glassware out of amber colored glass

Yellow glass
An easy way to make light yellow glass is by using silver.
For a deep yellow color the use of the cadmium is necessary. The problem with cadmium is that it is carcinogenic so it can not be used anymore.

Decolorization of Glass
Although we have shown you above which components are necessary to make colored glass the most precious for many people is colorless clear glass. 
Iron as an impurity in the raw materials, mostly in sand, causes an olive green color in glass. And there are other impurities which add a (slight) color, depending on where the raw materials come from. 

The glass makers have always tried to make clear, colorless glass and the first who were able to do that were the Venetians. The discovery of the use of manganese to decolorize glass took place in Venice in the 15th century. They were able to mask the greenish color caused by iron by adding a little manganese. Manganese became known as "glass makers soap" due to the ability to "cleanse" or neutralize the effects of iron impurities in the sand.      

Yellow glass plate (Lancaster Glass)

Selenium has more or less the same properties as manganese and adding a little bit of these materials makes glass light purple, which balances out the green color from the iron. Manganese was the most used decolorizer in the glassware industry from 1880 until the end of WWI. 
The use of open glass tanks instead of closed glass tanks switched the use to selenium as a decolorizing agent. 
Nowadays the decolorization is done mainly by adding sodium nitrate to the glass.

For thousands of years the glass makers have been busy to find ways to color glass and the search for new colors is still on. The big difference is that today the glass makers have all the information about the ingredients. Now they now that uranium is radioactive and that lead and cadmium are toxic. How different was that in previous times, when the glass makers were exposed to the dangerous chemicals. This gives also a limitation to the glass makers nowadays, to reproduce the exact colors which used to be produced two hundred years ago.

The production of colored glass and especially colored crystal glass is very laborious and costly. The industry has been looking around for alternatives. Glass is more and more colored by paints after the glass items are produced. Many different kinds of paint have been developed over the years. (For an interesting article about glass paints see: www.princessglassworld.com)  

Although clear glass is beautiful by itself, colored glass has amazed people for thousands of years and the thousands of years to come.