Substances Used in the Making of Coloured Glass

by

David M Issitt ~ Leading Expert on English Coloured Glass


The Basic Facts

From many years ago the glassmaker was adding substances to glass's raw material or batch to produce coloured glass. In those distant days it is more than likely he knew what happened but I doubt if he knew why. Many of the substances used in the making of coloured glass by our forefathers can no longer be used due to their properties, which contravene all aspects of health and safety in the workplace. Knowing what happened and not why, obviously prompted the fact colouring of glass was not a totally controlled operation. In many instances to achieve a specific colour was so much by trial and error, not as with his modern counterpart in today's modern high-tech glass making industry, who not only know the theoretical principles and properties of using varying substances to colour glass. It is the control usage of these substances along with other knowledgeable factors governing their properties, which achieves the final colour. Although not being a glassmaker or working within the industry, I admire very much the beauty portrayed in the colours which were developed under very hard conditions and much of my research into the colours of glass has been helped with by contributions from many who have and still work within the industry.

Davidson purple cloud glass,Purple slag glass and bue milk glass colours
cobalt blue cloud glass,uranium glass and vaseline glass colours Before we look at what substances give various colours to glass, certain basic factors affect the colouring power of some substances and these are the nature of the atoms and the action of both chemical and electrical forces. When we look at the glass colorants, which are used, it is only inorganic substances that are dealt with, therefore the nature of the atom is by far the most important factor. Many of the oxides or metals used can themselves be colourless or even highly coloured, depending on their state of valency. It is worthwhile to explain what valency is, 'It is the property of atoms or groups, equal to the number of atoms of hydrogen that the atom or group could combine with or displace in forming compounds'. Who would ever have thought that when my passion for collecting coloured glass would lead me into the world of chemistry and physics! My childhood days in the school laboratories come flooding back! So dependant upon how many links are available for combining with other atoms means that no clear line can be drawn between colouring and colourless substances. It is however known by glass makers in these days that certain compounds which contain an element with more than one valency give a more intensive colour when this element is in its lower state of valency, whilst other elements can give totally opposite results. This type of knowledge is used in decolourising glass, which must inevitably contain a small percentage of iron impurity. Iron oxide (FeO) in the form of ferrous iron will give a bluish-green tint to the glass, whereas in its higher state of oxidation as ferric iron, it gives a yellowish-green tint, which can be masked by the addition of complimentary colours such as red and blue. To ensure that most of any iron impurity is in its ferric state, oxidising agents such as sodium nitrate (NaNO3) are added to the batch in small amounts. The oxidation of iron is known as chemical decolourising whilst by masking the green with red or blue is called physical decolourising and is achieved by the addition of cobalt and selenium oxide.

There are also other factors, which have to be taken into consideration when producing Coloured Glass: -

1.  The temperature of the melt/batch Old print of glassmakers at work in a glasshouse
2.  Temperature of reheat during the working of the glass
3.  The temperature of the 'Lehr' (Annealing Oven)
4.  Duration of the melt/batch
5.  Time and temperature relationship at different stages in production
6.  The type of colorant being used
7.  Concentration of the colorant
8.  Atmosphere of the furnace
9.  The composition of the colorant within the glass composition, as is the case when iron is added to glass. The type of iron oxide formed decides if the glass will be blue or yellow
10.  The number of times the same glass is melted. Repeated melting of the cullet will usually give a darker tone to the finished piece.
Victorian Davidson amber glass vase and Crown Perfumery green glass perfume bottles - c.1880's The use of Iron (Fe)

Iron is a very useful and powerful colouring agent even though it can be an undesirable impurity in making glass. Iron when used in its highest state of oxidation could in combination with barium oxide (BaO) give a reddish-blue glass, but these would have melted under high oxygen pressures and cannot be produced in practice. Iron in its metallic forms cannot remain in equilibrium with glass and can be disregarded, however its ferrous and ferric forms are of a great help in producing coloured glass. In a reduced condition it can be combined with chromium to produce a deep green glass used in the production of wine bottles. Used with the combination of sulphur (S) , iron sulphides are formed giving a dark amber colour. Used on there own iron and sulphur would not give the amber colour required and a reducing agent such as carbon (C) powder is added to the batch. The shade of amber can only be controlled within narrow limits by varying the amount of coal, which is added in relation to the already existing iron impurity and the carbon matter in the raw materials.
What does Manganese do to glass? (Mn)

Some of the oldest compounds used in the colouring glass are manganese compounds. Evidence is found in early Egyptian purple glass that manganese is present. Manganese in its low state of oxidation is colourless, but it is a powerful oxidising agent and can be used for decolourising purposes to oxidise the iron content. Glassmakers have over the years substituted manganese by sodium nitrate or selenium in decolourising. Manganese is mainly used in the production of purple glass resembling the colour of potassium permanganate (KMnO4) crystals. The purple colour is achieved by the trivalent manganese however in its divalent state it only imparts a weak yellow or brown colour which are responsible for the green and orange fluorescence of manganese glass.
Davidson purple slag glass spill vase and slag glass jug - c.1880's
Victorian English green glass Fairy Lamp The use of Chromium (Cr)

Chromium is one of the most powerful of all colouring agents used in the glassmaking industry and is used in the production of dark green glass taking over from the use of iron oxide which had been used to produce this colour. The material can be introduced into glass either in the form of chromic oxide or potassium dichromate (K2Cr2O7) , the latter being a more convenient form. This material is a very powerful colouring agent that excessive use produces a black glass. According to glassmakers we now know that chromium is not easily soluble in glass and chromic oxide may form chromates, which remain in the glass as un-dissolved black specks. It was report that in the St. Helens area of Lancashire, England some railway wagons delivering limestone to the glassworks had previously carried chromium ore and minute quantities of the ore, which had not been swept out, had found their way into the glassworks and ruined many days of production. A costly error, which not only affected production but could have also lead to a lack of confidence in the finished product from the glassworks. Potassium chromate (K2CrO4) is yellow and this colour can be imparted to certain glasses. To produce emerald green glass in which a yellowish cast has to be avoided the addition of tin oxide and arsenic is necessary. The manufacture of chromium aventurine, which nowadays is hardly ever produced, is of historical interest. The aventurine effect is caused by the formation of fairly large plates of chromic oxide, which crystallise out from the melt. During the stage of blowing these crystals orient themselves nearly parallel to the glass surface and it is their reflections, which give a glittering effect to the finished article. Whilst chromium is associated mainly with the production of green glass, other colours from yellow through bluish-red, red to dark green or even black can be achieved in combination with other oxides.
Using Copper (Cu)

Copper is a very powerful and also a versatile colouring agent when used in colouring glass and its use can be traced back many years. The now famous Egyptian Blue Glass, which was so popular during the time of the Roman Empire, was made using a copper compound. Copper greens and blues are not difficult to produce, although the behaviour of copper in a silicate melt can be complicated. Copper was used most profusely to produce green glass. The art of using copper for ruby glass goes far back to ancient times but even so using copper oxide (CuO) to make ruby glass can be very difficult. Today we find copper being used to produce turquoise blue tones.
Davidson Turquoise Glass Sugar Bowl c.1880's
Early 19th century cobalt blue glass rolling oin and ale glass What effect does Cobalt have? (Co)

Cobalt is the most powerful blue colorant used in glassmaking producing rich blues when used in potash containing mixes, but it can also give shades of pink when used in a boro-silicate mix and green when used with iodides. There is no significant evidence as to when cobalt was first used as a colouring agent, but evidence can be seen in stained glass windows going back as far as the twelfth century. Cobalt is not only used in the glassmaking fraternity but was used extensively in the production of blue glazes in the pottery industry. Chinese porcelain, from the Tang Dynasty 616 to 906 and the Ming Dynasty 1368 to 1644, vases were decorated with cobalt blue. The addition of cobalt to the glass mix will produce a blue colour and its intensity depends upon the base glass. The deepest of blues are produced when used in glass containing potash. Very small quantities are used for physical decolourising, and the amount is so small that it must be added into the batch mix with sand, as the small amount of cobalt, if introduced on its own would have no chance of being uniformly distributed throughout the batch. In this way the sand acts as a pre-mixed dilutant. It is true to say most decolourising agents are used in very small quantities that it is normal to premix with sand to enable a better dispersion throughout the batch.
Nickel (Ni)

Nickel is not a very important colouring agent although it is used in the production of smoky coloured glass and in conjunction with cobalt for decolourising lead crystal. When it is introduced into lead crystal it gives a purplish colour, which compensates for a yellow tint produced by other constituents.
Tiffany contemporary lead glass crystal dolphin bowl
Davidson Uranium Glass /Vaseline Glass Pearline sugar bowl and cream jug c.1880's Uranium (U)

The thought of 19th century glassmakers using Uranium certainly emphasises the risks they undertook to achieve a piece of glass in a colour very desirable, unaware of the properties now associated with the handling of such a mineral. Glasshouses all over the world in the 19th century would surely have set high readings on 'Geiger' counters. Uranium produces a yellow coloured glass (This type of Uranium Glass is termed 'Vaseline Glass' by collectors in the USA), however when used in a very high lead containing glass will produce a deep red colour.
How Gold affects glass (Au)

The use of gold can be limited in its use by one factor alone, that of cost for the salts. There is also another point to consider when using gold and that is in controlling the conditions needed to produce the exact desired colour in the finished article. Gold gives glass a rich ruby colour and I am reliably informed by both Sam Thompson (Ex Stevens & Williams) and Stan Eveson (Ex Thomas Webb & Sons) that English glasshouses referred to this as Ruby Gold. Ruby gold is usually produced in a lead glass batch where tin (Sn) is present. A less dark rich ruby gold has become known as 'Cranberry', which has the same basic properties of its rich ruby relative. Many an ex-glassmaker will say it was merely by accident ruby gold was discovered, when a gold sovereign was tossed into the batch/melt by accident. Copper is an alternative to gold to produce ruby glass. The invention of gold ruby glass dates back to 1685 as noted in "De Auro", by Andreas Cassius, in which he describes for the first time the method of producing a red precipitate of stannic acid with gold which later became known as 'Purple of Cassius'. At the time the high price the glass commanded and the efforts need to make it could hardly be justified by its beauty. The principle techniques involved in producing red ruby glass are still based upon Cassius's discovery all those years ago.
Gold ruby glass bowl and Fenton cranberry glass pitcher
Silver is less used (Ag)

The use of silver to colour glass has not been as widely explored as other metals and oxides, although it will produce a variety of colours from brown to yellow. However, silver is mainly used for decorative purposes on glass.
Titanium (Ti)

Like silver, titanium oxide is very rarely used on its own as a colorant for glass, but it is used to intensify and brighten other glass colorants.

There are also many other substances used in making coloured glass which are not metallic elements and these also have an affect on colouring glass.


The use of Non-metallic Elements.

Other non-metallic elements are also used in the production of coloured glass, such as Phosphorus (P) ~ Selenium (Se) ~ Sulphur (S) ~ Tellurium (Te) of which Sulphur and Selenium are possibly the most important. Sulphur as we know is used with Iron (Fe) and Carbon (C) to produce amber glass, the colour of which can vary from very light straw to a deep reddish-brown or even black. Under the strongly reducing conditions created by the carbon, iron polysulphides are formed and these give the required depth of colour. In boro-silicate glasses containing a high proportion of boric oxide, sulphur can produce a pure blue colour and in combination with Calcium (Ca) in almost any glass it gives deep yellows. Cadmium (Cd) sulphides, which have a deep yellow colour, are often used in the production of glazes and enamels, but one has to remember that cadmium is a strongly toxic metal. National legislation has over the years been enforced in many countries limiting the use of cadmium. Such advancements in the control on the use of such toxic metals was never in place during the Victorian era of glassmaking and one can only wonder what affects using such metals caused the worker. Selenium is one of the most important colouring agents for making pink and red glass. Being a non-metallic element, selenides, selenites and selenates of metallic elements are formed, but most of these are colourless and it is the free selenium atoms, which give the pink colour. Selenide produces a range of deep red colours. One can see that if the amounts of selenium are increased and added to a cadmium sulphide glass, the pure yellow colour changes to orange and finally to a brilliant red known as 'Selenium Ruby'. It depends on the composition of the base glass and furnace atmosphere, whether ruby glass can develop their colour on melting or if the colour has to be stuck by reheating the cooled glass.

The legacy of producing coloured glass continues.

It is hard in this day and generation to fully appreciate the hazards that must have existed in glasshouses throughout the world during the 19th century, when all glassmakers and glasshouses were experimenting with colours. From those far-gone days, we still see the current generation of glassmakers testing and trying new colours. However, today they are controlled over what substances can and can't be used and because of this it will never be possible to reproduce the exact colours produced those many years ago. The glass industry has not lost the art of producing fine coloured glass and when one looks at some of the newer items coming onto the market, we see the new glassmaker still admires the workmanship of their forefathers, in both the designs and colours they now produce. I sincerely hope that this insight to what basic substances were and are used to produce that coloured piece of glass you so value will help you to realise how that colour was made. May you continue to enjoy the many facets of collecting coloured glass with the knowledge of what you hold in your hands.

David M Issitt

This article is copyright protected and cannot be used
in any form without the expressed permission of
David M. Issitt the author.
Copyright 2005

The illustrations within this document are copyright Tony Hayter (1st.Glass) 2008


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