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.
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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.
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There are also other factors, which have to be taken into consideration when
producing Coloured Glass: -
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1.
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The temperature of the melt/batch
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2.
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Temperature of reheat during the working of the glass
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3.
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The temperature of the 'Lehr' (Annealing Oven)
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4.
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Duration of the melt/batch
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5.
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Time and temperature relationship at different stages in production
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6.
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The type of colorant being used
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7.
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Concentration of the colorant
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8.
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Atmosphere of the furnace
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9.
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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
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10.
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The number of times the same glass is melted. Repeated melting of the cullet
will usually give a darker tone to the finished piece.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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The author of this article may be contacted at:
glasswriter@crowfans.net
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) ©
Copyright 2005 ©
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