Viscosity of Colours

“I have been advised in the past, that blue fires quicker. I was told this by a Master glass maker.”

Viscosity has some relation to colour and intensity.  But you should note black & stiff black are both of the same intensity, and are fusing compatible, but have different viscosities.  This shows that colour is not the only determinant of viscosity, as the stiff black shows the viscosity can be adjusted within the same colour.  The quotation above indicates that the reasons behind any declarative statements need to be investigated.

Some factors in viscosity

Opalescent colours tend to be more viscous than their transparent counterparts.

It is the metals that develop the colours that produce much of the difference in viscosity.  The same metal can produce different colours in different furnace conditions, so viscosity cannot be assumed to be directly related to colour. 

Some people in the past have done their own tests on viscosity and colour relationships, but I have no access to them.  More recently Bob Leatherbarrow shows (Firing Schedules for Kilnformed Glass, 2018, chapter 7.2.5, p.88) some slumping tests done with opalescent glass. It shows how much less viscous black is than white, and that white is the most viscous.  The other results show red a little less viscous than white, then some greens, yellows and oranges, other greens, purple, pinks (in that order) and of course, the least viscous is black.

Transparent glasses tend to be less viscous than opalescent glasses.

How does this information relate to kilnforming practices?  It indicates that a piece with the less viscous glasses requires lower temperatures or less heat work to complete the forming of the glass than more viscous glasses.

When you have a combination of more and less viscous glasses in a piece you need to fire more slowly to ensure all the glass is thoroughly heated through and will deform equally.  You will need to observe and be prepared to move the piece on the mould to straighten it up.

Do your own viscosity tests

You can do your own tests for viscosity differences by arranging 10mm wide strips all the same length (about 30cm) of different colours. These should be placed on a kiln washed pair of narrow batts set parallel to each other 25cm apart and about 15cm high.  Fire at about 150°C per hour to about 650°C, setting the soak to 30 minutes.  Observe at intervals from 620°C.  Stop the firing when the least viscous has almost touched the floor of the kiln. When fired all together at the same time you can see the relative viscosity of the colours tested.  You can label these and store them, or tack fuse these labelled curves to a piece of base glass for future reference.

Colour Dilution of Powders

Sometimes you do not have a tone or shade of a colour you need for your project.  Other times you want to have a gradation of shade across a piece.  There is the obvious solution of mixing a colour with clear to produce lighter shades.  But there is a difficulty when mixing clear with powders to fuse. The result is often a pointillist effect with points of light coming through the colour. There are several approaches to this difficulty.

One way is to use a powder made from a tint of the colour.  But sometimes there is not a tint made. Sometimes you do not have that tint in stock. So, you must look to other solutions.

Credit: www.warm-glass.co.uk

An alternative is to use clear powder to mix with the intense colour you want to dilute.  You will need to test varying proportions of clear to colour to get the tone you need.  You may be surprised at the amount of clear needed.  And there still is the slight possibility of points of light coming through the clear.

Another possibility is to use one of the less dense white powders to mix with the colour.  White powders such as the Bullseye 000243, translucent white, or the 000113, dense white are possible.  The very dense or lacy whites are not as suitable. One is too opaque, the other is uneven in colour. Again, testing will be required, and you may be surprised at how little is required to alter the tone.

One other way I have used is to mix fine frit with the powder.  This has less control than the other methods but can provide significant dilution of the intense colours.  If you want to see if this is suitable, you can follow this process. 

Add a few drops of water to the clear frit in a small container. Close it and shake to get all the frit coated with a film of water. If after shaking the frit is not “clumping” you can add a little more. Too much water will create a slurry which is not suitable.  So, add only a small amount of water at a time until the frit is like damp sand on the beach. Any excess water must be poured off. 

Add powder to the damp frit, and shake well again to coat the frit with powder. If the frit does not seem to be fully coated, add a little more powder.  The film of water on the frit allows the powder to adhere temporarily to the frit.  

This mixture can then be applied to the surface and smoothed with a pallet knife. This will not guarantee there are no clear pinpoints, but it will reduce them to a minimum. You will not have the subtle differences in tone that sifting powder can give you, but it is a cost-effective way of diluting intense powder colours that can have advantages over mixing powders.

Of course, the various methods of diluting colour described here can be used to combine powders to produce new colours.

Iridescence

What is it?       How permanent is it?

“Many special effects can be applied to glass to affect its colour and overall appearance. Iridescent glass … is made by adding metallic compounds to the glass or by spraying the surface with stannous chloride or lead chloride and reheating it in a reducing atmosphere.” 

Older glass can appear iridised because of the light reflection through the layers of weathering.

“Dichroic glass is an iridescent effect in which the glass appears to be different colours, depending on the angle from which it is viewed. This effect is caused by applying very thin layers of colloidal metals (e.g., gold or silver) to the glass.”

Source: https://www.thoughtco.com/

A rainbow iridescent appearance caused by an oil film on water is seen by light being reflected from both the top oil surface and the underlying water surface.  The light reflected from these two surfaces or boundaries have slightly different wave times and so interfere with each other to create this colourful pattern.

This is also observed in soap bubbles.  Here the light is reflecting from both the inner and outer surfaces of the film.

This iridescent appearance is termed thin-film interference.  It is an occurrence in nature where there is a thin film through which light can penetrate and so reflect off the surfaces of the film.  These surfaces are termed boundaries where the light can reflect. 

The thickness of the film can enhance or reduce the iridised effect. 

At a certain thickness the light waves reflected can cancel each other out.  This is described as a destructive interference pattern as it reduces the reflection.  The phenomenon can be used to provide non-reflective surfaces.

At other thicknesses there is an iridised effect.  This is caused by the reinforcement of the recombination of the two light waves reflecting in phase or nearly so.

Control of the thickness can give the silver or the gold iridised appearance, as in the Bullseye iridised glasses, in addition to the rainbow and other effects.

The nature of the light affects the colours of the iridescence.  If the light is daylight or similar it is a combination of many wavelengths.  The different wavelengths reflecting from the “boundaries” or surfaces provide the multiplicity of colour.  If the film has variations in thickness, there will be variations in the colours created.

A diagram from Wikipedia shows how the reflections work at the microscopic level.

https://en.wikipedia.org/wiki/Thin-film_interference

The permanence of the film causing the iridisation appears to be dependent on the metals used and the way in which they are deposited.

Combining Black and White

Black and white are at almost opposite ends of the viscosity spectrum in glass terms. Black is the runniest, and white is the stiffest. Black transmits heat more quickly to the lower layers than white.  White is the glass that absorbs and transmits heat most slowly.

an example from Pintrest

A lot of care is required when combining the two.

If the white is on top of black, the white shades the heat – more than other colours - from the black underneath, so a lot of stress can build up in the black.

You need to give a lot of time for the two to adjust to each other. A slower rate of advance than normal is advisable. A significantly longer soak at annealing temperature is required. The annealing cool needs to be much slower than for other glass of the same dimensions.  Consider slowing the rates to half your normal firing rates.  Also double your soak times.  After some experience you will be able to alter these cautious rates to those more suitable for you.

Striking glass

Yes, much glass is striking in its effect.  But the term is used in a technical sense to indicate the glass has not reached its intended colour without further firing.

A striking glass is one that changes to its true colour. Not one which takes up a different colour.  There seem to be differing ideas on how striking works, but it is an intentional process.

Several glasses coloured with copper or silver strike to Their final colour when heated.  It seems that copper when used to make red (rather than blue or green) can undergo a chemical change during the heating.  The copper oxide used is normally Cu₂O.  When heated the copper and oxygen molecules can separate and form bonds with other molecules.  The rapid cooling that is used in glass prevents the copper and oxygen from combining in the Cu₂O formation.  The extent of this dissociation determines the degree of colour change.  Thus, the colour is affected by the heat work given to the glass – assuming the starting proportions of materials are the same.  This can occur with some other colouring metals too.

Another form of striking is caused by the growth of crystals within the glass. In these cases, usually in silver bearing glass, the metals separate from the silica and form small crystalline structures which are also fixed by the rapid cooling required for glass.

There is another theory that the colour change is due to the orientation of the colouring molecules within the glass matrix.  The idea is that the molecules will change from the clearer state to the struck colour due to the orientation caused by reheating and cooling.

The actual process seems to be unknown in a definitive sense.  What is known is that temperature, a reducing or oxidising atmosphere, and heat work will vary the intensity of the strike in colour.  This means that where the project is especially sensitive, you must undertake experiments to help predict the colour that will be achieved with the conditions you choose to use.

Specific Gravity, CoLE, and Colourants of Glass

I’ve been asked the question “is there is differential in specific gravity as related to COE or colorant used in the glass (white opal v clear)”? 

Using the typical compositions of soda lime glass (the stuff we use in fusing), both transparent and opalescent and combining the specific gravity of the elements that go to make up the glass, I have attempted to answer question - the last part of the question first.

Difference in specific gravity between transparent and opalescent glass

Transparent glass

Typical transparent soda glass composition % by weight (with specific gravity) Material                         Weight        S.G. Silicon dioxide (SiO2)           73%         2.648 Sodium oxide (Na2O)            14%         2.27 Calcium oxide (CaO)               9%         3.34 Magnesium oxide (MgO)          4%         2.32 Aluminium oxide (Al2O3)          0.15%    3.987 Ferrous oxide (Fe2O3)               0.1        5.43 Potassium oxide (K2O)             0.03       2.32 Titanium dioxide (TiO2)            0.02        4.23

There are, of course minor amounts of flux and metals for colour in addition to these basic materials.

The specific gravity of typical soda lime glass is 2.45.

Opalescent glass

Initially opalescent glass was made using bone ash, but these tended to develop a rough surface due to crystal formation on the surface.  The incorporation of calcium phosphate (bone ash) and Flouride compounds and/or arsenic became the major method of producing opalescent glass for a time.

The current typical composition by weight (with specific gravities) is:

Silicon Dioxide (SiO2) –             66.2%,     2.648 SG

Sodium Oxide (Na2O) –            12%,        2.270

Boric Oxide (B2O3) –                10%,        2.550

Phosphorus pentoxide (P2O5) –  5%,         2.390

Aluminum Oxide (Al2O3) –         4.5%,      3.987

Calcium oxide (CaO) –              1.5%,      3.340

Magnesium oxide (MgO) -         0.8%,      2.320

The combined specific gravities are within 0.03% of each other -  a negligible amount.  So, the specific gravity of both opalescent and transparent glass can be considered to the same. For practical purposes, we take this to be 2.5 rather than the more accurate 2.45.

Other glasses exhibit different specific gravities due to the materials used, for example:

Lead Crystal Glass

Lead Crystal glass contains similar proportions of the above materials with the addition of between 2% and 38% lead by weight.  Due to this variation the specific gravity of lead crystal is generally between 2.9 and 3.1, but can be as high as 5.9.

Borosilicate glass

Non-alkaline-earth borosilicate glass (borosilicate glass 3.3)

The boric oxide (B₂O₃) content for borosilicate glass is typically 12–13% and the Silicon dioxide (SiO₂) content over 80%. CoLE 33

 

Alkaline-earth-containing borosilicate glasses

In addition to about 75% SiO₂ and 8–12% B₂O₃, these glasses contain up to 5% alkaline earths and alumina (Al₂O₃).  CoLE 40 – 50

 

High-borate borosilicate glasses

Glasses containing 15–25% B₂O₃, 65–70% SiO₂, and smaller amounts of alkalis and Al₂O₃

All these borosilicate glasses have a specific gravity of ca. 2.23

Correlation between CoLE and and specific gravity?

This comparison of different glasses shows that the materials used in making the glass have a strong influence on the specific gravity.  However, there does not appear to be a correlation between CoLE and specific gravity in the case of borosilicate glass.  If this can be applied to other glasses, there is no correlation between specific gravity and CoLE.

Correlation between specific gravity and colourisation minerals and CoLE?

The minerals that colour glass are a very small proportion of the glass composition (except copper where up to 3% may be used for turquoise).  The metals are held in suspension by the silica and glass formers.  That means the glass is moving largely independently of the colourants which are held in suspension rather than bring part of the glass structure. There is unlikely to be any significant effect of the metals on the Coefficient of Linear Expansion.  The small amounts of minerals are unlikely to have an effect on the specific gravity.  So, the conclusion is that there is no correlation between CoLE, specific gravity, and colouring minerals.

The short answer

This has been the long answer to the question.  The short answers are:

·         The specific gravity of soda lime transparent glass and opalescent glass is the same – no significant difference is in evidence.

·         There appears to be no correlation between specific gravity and CoLE.

·         There is unlikely to be any correlation between colourant minerals and CoLE or specific gravity.

Adding Colour to Slumped Pieces

Sometimes an already fused and slumped clear piece needs colour for interest, definition, etc. The problem is how to do it without altering the fused piece.

You can use cold paints. Both acrylic and oven baked paints can be applied, but often they are unsubtle, harsh colours.  These are not permanent.

You can apply enamels.  These can be the ones produced for glass fusing, if fired carefully. The curing temperature of 780°C means that you need to fire slowly to about 600°C and then quickly to 780°C with no soak and AFAP to annealing.  This fast rate of advance is to preserve the shape as much as possible at temperatures above that required for slumping. This will need to be done in the mould, of course.

You can more safely use traditional glass stainer colours, which are also called enamels, although they are slightly different from the traditional ones.  These cure between 520°C and 580°C so can be fired as normal in one steady climb to the top temperature with no soak and quickly down to annealing. To be sure the shape is retained, the glass should be in the mould during the firing.

Use of frits and powders requires the higher temperatures that will distort the piece unless fired in the mould. When firing to tack fuse in a mould, you need to be careful that you do not damage the mould during the higher temperature firing, nor get the separator incorporated into the powder.  If you can place the powder or frit on top of the glass, you will get a better result at a lower temperature as the frit will heat and spread more easily on top than on the bottom. 

In general, it is not recommended to add colour to slumped pieces with frits and powders.  It is hard on the mould, and risks the glass sticking to the mould. Even if successful, the slumping mould will have to have the existing kiln wash removed and new added to avoid the kiln wash sticking to the next piece to be fired.  

It is better to slump the piece to flat, if possible, and then add the frits and powders before fusing.  Then slump again.



These notes show that it is important to assess the flat piece critically before proceeding to the slump.  This can mean setting the piece aside for a few days to review your impression of the fusing result.  This little time elapse can give you a fresh view of what the piece requires, if anything. 

Copper inclusions

Inclusions of metals can be achieved with care.  Copper is a very good metal, as it is soft, even though its expansion characteristics are very different from glass.  This note provides some things you might consider when planning to include copper in your fused pieces.

The copper sheet should be stiff, but not thick. If the metal can be incised with a scribe and maintain that through gentle burnishing, it is suitably thick. The usual problem is that the copper is too thick rather than too thin.  Copper leaf can be very faint if a single layer is used.  Placing several layers of leaf improves the colour, but often provides wrinkles.  In summary, the requirement is to get a thickness of copper that will retain its structure, but not be so thick and stiff as to hold the glass up during the fusing process.  

Do not use the copper foil as used for stained glass applications. The adhesive backing produces a black colour from the adhesive and many bubbles -  sometimes a single large one.

Copper can provide several colours.

Copper sheet normally turns burgundy colour when oxidised.  This means that there is enough air reaching the copper to oxidise it to deep copper red.  This most normally happens, because a lot of air can contact the metal during the extensive bubble squeeze usually given to inclusions.

To keep the copper colour, clean the metal well metal well with steel wool or a pot scrubber. If you use steel wool, wash and polish dry the metal before fusing.  Reduction of air contact with the metal helps to retain the copper colour.  There are two methods I have used.  Addition of a glass flux like borax or other devitrification spray will help prevent the air getting to the surface.  Another method of avoiding oxidisation, is to cover the copper with clear powdered frit, as well as the surrounding glass.

In certain circumstances you can get the blue green verdigris typical of copper in the environment.  This is an extent of oxidisation that is between the clean copper coloured metal and the burgundy colour of extensive oxidisation.  The key seems to be be a combination of restricted air supply, shorter bubble squeezes and lower temperatures.  Experimentation is required to achieve this consistently.


The spaces under and over the copper give the opportunity for bubbles to form. 

This means that the copper needs to be as flat as possible for one thing.  Burnishing the copper can have a good effect on reducing the undulations in the copper.  Thinner copper is easier to make flat than thicker.  If you can stamp a shape from the copper with a stamper designed for card making, it is a good indication that it will burnish flat.  Thicker copper sheet holds the glass up long enough in the temperature rise during the bubble squeeze to retain air around the metal.  This remains the case even after burnishing to be as flat as possible.

The second element that can help to reduce bubbles around the copper is to sprinkle clear powder over the copper sheet once in place on the glass.  The spread of the powder over the glass assists in giving places for the air between layers to escape.

These two things combined with a long slow squeeze can reduce the amount of bubbles you get.  It cannot totally eliminate them.

Of course, a longer bubble squeeze allows air to be in contact with the copper and promotes the change to a blue green or burgundy colour.

Light and Dark in Designs

Chiaroscuro – This word borrowed from Italian ("light and shade" or "dark") refers to the modelling of volume by boldly contrasting light and shade. 

Glass artists need to be very cognisant of light and dark, both in terms of colour selection and in terms of density. A very thick dense glass of a dark shade of any colour will create a much more intense darkness than glass that is thinner and less dense.

In terms of colour, lighter hues go where the sun shines or where the eye is to be drawn. Pastel shades indicate brightness and light. Within some opalescent and art glasses it is possible to find a shade of colour graduating to white or light yellow. 

Shading can be achieved by using the white areas to indicate where light is falling. A denser dark glass can be used to indicate where light does not fall, or where very little light can filter through. It can also play the part of negative space.

Sometimes, it is useful to use a monochrome scheme to assist in determining where the light and dark should be, as in this pear:

The contrast between light and dark can be used in several ways. Darkness can indicate depth of field or distance when used in a general landscape. Or, it can be used to bring a foreground out, making other elements more vivid.

The key thing to remember in using stained glass is to not be afraid of dark glasses. They can very useful, even if of very odd hues of colour.

Colourising Metals for Glass

There are many minerals and chemicals that are used to give glass its variety of colour.  This note attempts to give information on the most common elements and combinations used to impart the colours to the glass.

Antimony oxides produce white glass as do tin oxides.  Together with lead, antimony results in yellow.

Cadmium together with sulphur forms cadmium sulphide and results in deep yellow colour.  Together with selenium and sulphur it yields shades of bright red and orange. 

 

Chromium is a very powerful colourizing agent, yielding dark green or in higher concentrations even black colour. Together with tin oxide and arsenic it gives emerald green glass. Chromium aventurine, in which aventurescence was achieved by growth of large parallel chromium(III) oxide plates during cooling, was also made from glass with added chromium oxide in amount above its solubility limit in glass.

The material can be introduced into glass either in the form of chromic oxide or potassium dichromate, the latter being a more convenient form.  Potassium chromate is yellow and this colour can be imparted to certain glasses. To produce emerald green glass in which a yellowish cast must be avoided, the addition of tin oxide and arsenic is necessary.

Chromium is associated mainly with the production of green glass, but other colours from yellow through bluish-red, red to dark green or even black can be achieved in combination with other oxides.

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 green when used with iodides.  The deepest of blues are produced when used in glass containing potash.

Small concentrations of cobalt (0.025 to 0.1%) yield blue glass. The best results are achieved when using glass containing potash. Very small amounts can be used for decolourizing. Addition of 2% to 3% of copper oxide produces a turquoise colour.

Copper is a very powerful and versatile colouring agent when used in colouring glass.  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.  

Pure metallic copper produces a very dark red, opaque glass, which is sometimes used as a substitute for gold in the production of ruby-coloured glass. The art of using copper for ruby glass goes far back to ancient times but even so using copper oxide to make ruby glass can be very difficult. Today we find copper being used to produce turquoise blue tones.

Didymium gives green colour (used in UV filters) or lilac red.

Metallic gold, in very small concentrations (around 0.001%, or 10 ppm), produces a rich ruby coloured glass (gold ruby), while lower concentrations produces a less intense red, often marketed as cranberry. The gold is used as gold chloride. The colour is caused by the size and dispersion of gold particles.

Iron is a very useful and powerful colouring agent even though it can be an undesirable impurity in making glass. Iron in its metallic forms cannot remain in equilibrium with glass and can be disregarded. But its ferrous and ferric forms are of a great help in producing coloured glass.  Iron(II) oxide may be added to glass resulting in bluish-green glass.  

Together with chromium it gives a richer green colour.  In a reduced condition, it can be combined with chromium to produce a deep green glass. 

Used with the combination of carbon or other reducing agents, sulphur and iron sulphides give a dark amber colour.

Lead compounds produce a range of yellows.

Manganese can be added in small amounts to remove the green tint given by iron, or in higher concentrations to give glass an amethyst colour.

Manganese dioxide, which is black, is used to remove the green colour from the glass. This results in a very slow chemical process where it is converted to sodium permanganate, a dark purple compound. Windows made with manganese dioxide solarise to change to a colour which is lightly tinted violet because of this chemical change.

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.  Manganese is mainly used in the production of purple glass resembling the colour of potassium permanganate crystals. The purple colour is achieved by the trivalent manganese however in its divalent state it only imparts a weak yellow or brown colour.

Nickel, depending on the concentration, produces blue, or violet, or even black glass. It is used in the production of smoky coloured glass

Lead crystal with added nickel acquires purplish colour. Nickel together with a small amount of cobalt can be used for decolourizing of lead glass.  When it is introduced into lead crystal it gives a purplish colour, which compensates for a yellow tint produced by other constituents.

Selenium, like manganese, can be used in small concentrations to decolourize glass, or in higher concentrations to impart a reddish colour, caused by selenium nanoparticles dispersed in glass. It is a very important agent to make pink and red glass. When used together with cadmium sulphide, it yields a brilliant red.

Silver compounds such as silver nitrate and silver halides can produce a range of colours from orange-red to yellow. The way the glass is heated and cooled can significantly affect the colours produced by these compounds.

Sulphur, together with carbon and iron salts, is used to form iron polysulphides and produce amber glass ranging from yellowish to almost black.  With calcium it yields a deep yellow colour.  

In borosilicate glasses rich in boron, sulphur imparts a blue colour. Cadmium sulphides, which have a deep yellow colour, are often used in the production of glazes and enamels.

Tin oxide with antimony and arsenic oxides produce an opaque white glass (milk glass), first used in Venice to produce an imitation porcelain.

Adding titanium produces yellowish-brown glass. Titanium, rarely used on its own, is more often employed to intensify and brighten other colourizing additives.

Uranium (0.1% to 2%) can be added to give glass a fluorescent yellow or green colour.  Uranium glass is typically not radioactive enough to be dangerous.  It is often referred to as Vaseline glass by USA collectors.  When used with lead glass with very high proportion of lead, produces a deep red colour.

Chart

A good visual chart of minerals and resulting colours is here.

Influence of the glass-making process on colour.

It is not only the minerals that give the glass the colour; it is combined with the way in which the materials are treated.  The physical conditions under which the glass is made also have an influence on the colour.  The main ones are:

1.   The temperature of the melt/batch

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