Lustres

Lustres are metallic colourants in colloidal suspension. They provide intense reflective colour. They are most effective when used sparingly as accents. They are supplied as a dark brown viscous liquid in small bottles. They are widely available from ceramics suppliers.

A bar with gold lustre. Credit: Bath Potters

 The application of these is important, and not only because they are expensive.

 They must be applied to clean dry surfaces with a smooth brush. The gold, platinum, copper, and bronze lustres do not need dilution. The brush should not have a lot of lustre, nor too little. Too much causes burning, flaking, dullness or clouding during and after the firing. The application must be uniform in thickness. “Application of lustres is possibly the most important factor in achieving the best results. The more richly coloured lustres require a fairly thin coat while other lustres (particularly the metallic lustres) require an even thinner coat.” Bath Potters.

silver lustre brushed on.  Credit: Pottery Crafts

 The kiln should be vented until the carrier has burned off. The absence of the smell will indicate when this has been achieved. The firing of lustres can be between 586°C/1088°F and 733°C/1353°F. Metallic lustres usually fire between 586°C/1088°F and 617°C/144°F. These lower ones are in the slumping range of temperatures and can be applied to the flat blank before slumping or to the completed flat blank depending on the requirements of the lustre.

What are enamels?

Not all enamels are equal

Enamel paints

This description refers to a paint that air dries (or with minimal heat) to a hard finish (usually gloss). Most commercially available enamel paints are significantly softer than either vitreous enamel or heat cured synthetic resins. The term "enamel paint" generally is used to describe oil-based covering products, usually with a significant gloss finish. Many latex or water-based paints have adopted the term.

Enamel paint has come to mean a "hard surfaced paint" and usually is in reference to paint brands of higher quality, floor coatings of a high gloss finish. Most enamel paints are alkyd resin based. Some enamel paints have been made by adding varnish to oil-based paint. Enamels paint can also refer to nitro-cellulose based paints. Nitro-cellulose enamels are also commonly known as modern lacquers.  These have been largely replaced by synthetic coatings like alkyd, acrylic and vinyl.

More information at: https://en.wikipedia.org/wiki/Enamel_paint

Enamel paints are used for coating surfaces that are outdoors or otherwise subject to hard wear, or variations in temperature.  A widespread application is in paints for cars. It is also used frequently to decorate or label bottles due to the low curing temperatures of some formulations.

Vitreous enamels 

Vitreous enamels are used in a variety of circumstances.  Metal signs are most frequently enamel coated; they are used in ceramics as over glazes;  and they are used on glass in many circumstances.

Vitreous Enamel is simply a thin layer of glass fused at high temperature on to the surface of a metal or glass. Vitreous Enamel can be defined as a material which is a vitreous solid obtained by smelting or fritting a mixture of inorganic materials.  The word enamel comes from the High German word ‘smelzan’ and from the Old French ‘esmail’.

The key ingredient of vitreous enamel is finely ground glass frit. Colour in enamel is obtained by the addition of various minerals and metal oxides. 

Vitreous enamel is made by smelting naturally occurring minerals, such as sand, feldspar, borax, soda ash, and sodium fluoride at temperatures between 1200°C and 1350°C  until all the raw materials have dissolved. The molten glass which is formed is either quenched into water or through water-cooled rollers. This rapid cooling prevents crystallisation. The resulting frit is ground in a rotating ball mill either to produce a water-based slurry or a powder.

At the milling stage, other minerals are added to give the properties and colour required of the final enamel. Different enamel colours can be mixed to make a new colour, in the manner of paint. Enamel can be transparent, opaque or opalescent.

More information at: 

https://www.vea.org.uk/what-is-enamel/

https://en.wikipedia.org/wiki/Vitreous_enamel

https://www.iom3.org/vitreous-enamellers-society/vitreous-enamel-information

Metal enamelling

Modern frit for enamelling steel is typically an alkali borosilicate glass with a thermal expansion and glass temperature suitable for coating steel and other metals. Raw materials are smelted together between 1,150 and 1,450°C (2,100 and 2,650°F) into a liquid glass that is directed out of the furnace and thermal shocked with either water or steel rollers into frit. Vitreous enamel is often applied as a powder or paste and then fired at high temperature. This process gives vitreous enamel its unique combination of properties. The smooth glass-like surface is hard; it is scratch, chemical and fire resistant. It is easy to clean and hygienic.  It all started 3500 years ago in Cyprus. Since 1500 BC, enamelling has been a durable, attractive and reliable material.

More information at: 

https://en.wikipedia.org/wiki/Vitreous_enamel

https://www.vea.org.uk/what-is-enamel/ 

Enamels in Ceramics

Overglaze decoration, overglaze enamelling or on-glaze decoration are all names for the method of decorating pottery, where the coloured decoration is applied on top of the already fired and glazed surface, and then fixed in a second firing at a relatively low temperature.  The colours fuse on to the glaze, so the decoration becomes durable. This decorative firing is usually done at a lower temperature which allows for a varied and vivid palette of colours, using pigments which will not colour correctly at the high temperature necessary to fire the clay body.

More information at:  https://en.wikipedia.org/wiki/Overglaze_decoration

Glass Enamels

Glass enamels are produced in the same way as enamels for metals and ceramics.  The frit characteristics are adjusted for various applications and temperatures.  This combination of finely ground frit and metals for colouring are often combined with a binder or carrier medium.  It is similar to vitreous enamel on metal surfaces, but the supporting surface is glass. It is also close to "enamelled" overglaze decoration on pottery, especially on porcelain, and it is thought likely that the technique passed from metal to glass (probably in the Islamic world), and then in the Renaissance from glass to pottery (perhaps in Bohemia or Germany). 

Glass may be enamelled by sprinkling a loose powder on a flat surface, painting or printing a slurry, or painting or stamping a binder and then sprinkling it with powder, which will adhere.  The powdered frit can be in the ceramic on-glaze composition suitable for fusing or casting temperatures, or it can be adjusted for slumping temperatures as in the traditional glass stainers’ enamels. It can produce brilliant and long-lasting colours, and be transparent, translucent or opaque. Generally, the desired colours only appear when the piece is fired, adding to the artist's difficulties.

More information at:  https://en.wikipedia.org/wiki/Enamelled_glass

The term enamel is applied to a wide variety of coating materials.  The range of usage is indicated, and the manufacture and applications of vitreous enamels is indicated.  The term enamel is not properly applied to finely ground coloured glass in a medium.

Polishing Panels with Paint and Enamels on the Glass

Do not use black patina on the lead/solder lines on your finished work if there is any painted glass into the panel whether leaded or foiled. This relates to traditional painting on glass, using vitreous paints, fired at ca. 670C and to cold paints often cured in a domestic oven.

When using enamels within the painting, do not let any patina come in contact with the finished production. The patina will etch out all the enamel colour. The patina will etch off the outer layer, either removing the shiny top layer of paint, or the delicate lines of detail work altogether. Copper patina takes a little of the paint off, but not nearly as badly as the black.

Instead, brush the panel with a natural bristle brush, as used for putty clean up, and polish.

Silver stains that have been properly held at the maturing temp, should withstand any patina application, as they have become incorporated at the molecular level with the glass.

Kiln Elements - Aging

As elements age, they generally increase in their resistance. This increase in resistance decreases the amount of amperage and, so, the amount of heat given off by the elements. This explains  why older kilns sometimes go so slowly and may not reach their maximum temperature.

There are several factors which affect the longevity of elements and so have implications for firing practices.

• ·        Contaminants such as silica which is contained in kiln wash and some glazes attack the aluminium oxide coating of the wire.
• ·        Allowing the wires to become tightly wound increases overheating of sections of the element.
• ·        Powders, paints and kiln wash accidentally touching the elements cause rapid corrosion of the elements if not cleaned off before firing.
• ·        Firing close to the elements allows fumes to contact the elements.
• ·        Subjecting elements to reducing atmospheres will age the elements quickly.  This would be done by introducing organics or oils into the kiln without venting.  Among the things that will attack the aluminium oxide coating of the elements are carbon, wax, halogens (such as chlorine or fluorine), molten metals (such as zinc, aluminium), lead glazes, alkaline metals, borax compounds.

All these elements attack the element coating.  And each time you fire the slight difference in expansion between the core of the wire and the coating creates cracks in the coating.  The exposed core forms new coating to fill the gaps.  This over time reduces the thickness of the element wire.  As the wire thins, the resistances increases, causing more fissures in the coating to occur, accelerating the aging process.

The next in this series is about how firing practices can affect the life of elements.
Firing Practices

Other relevant posts:
Nature of elements
Maintenance

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. 

CoE of Paints for Fusing

CoE of the glass carrier for paints is a distraction.

Paint has been applied to glass and fired for at least seven centuries – long before CoE measurement.  The earliest enamels were intensely coloured glass powders applied to depressions in the base metal (iron, gold, copper, brass, etc) and heated.  More detailed images began to be created when the powers were mixed with a liquid binder and painted on either in a single, or multiple layers onto glass and metals.

Silver stain became popular in the 16^th century and has continued since.  This is a different way of colouring the glass, as the colour does not laminate with the surface, but is chemically combined with the glass.  Various silver salts produce different colours and vary in intensity at different temperatures.  This can provide a variety of effects at fusing temperatures where it “metalises”, providing ambers and blues.

CoE in relation to paint does not matter.

The amount of paint is miniscule in relation to the mass of glass to which it is applied, and so any incompatibility would not have sufficient strength to break the glass. If the paint’s glass carrier was too incompatible, it would come off instead of breaking the glass, in any case.

The composition of the fusing glass paints is largely unknown, although commonly supposed to be powdered glass frit. Some may be the same as enamels used in metal enamelling. Some others may be the same as the on-glaze ceramic colours. They all have glass as the carrier of the colour.  Still, the amounts of glass involved are very small and compatibility is not a concern.

In any case CoE is not the most important element in compatibility.

CMC as a Medium

There is a lot of use of powdered glass in a medium supplied in an applicator bottle.  This is convenient and expensive.  You can approach the consistency of the commercial product by use of CMC

CMC is carbyl methyl cellulose. It is a binder (glue) and it helps keep solids suspended in liquid. You can buy it at a pottery supplier as it is often used in glaze mixtures and known as a glaze binder. It is also used a lot in the food processing industry and is available as food grade CMC.  It normally is supplied in powder form.

To make use of it you need to dissolve a teaspoon or two in a half pint of water and let it stand. It does take a while to wet and in doing so it expands. By the next day it will be a thick mass. It can be further diluted, but it is often best to make it to the consistency you will need for the current purpose.  It will require a little experimentation to get the proportions right for various purposes.  

Put on a dust mask. Sift the powdered glass to remove the coarsest particles and mix the result with the CMC, 1 to 2 of powder or more.  Mix thoroughly and leave to stand overnight. Then dilute to the desired flow if required and mix well. Again let it stand overnight to completely incorporate the powder.  If it is too liquid, it is desirable to add more powder than more CMC to avoid diluting the colour.

Float Glass

A question about sharp raised points on the corners of a square bubble plate made of window glass is the occasion to discuss some characteristics of float glass. 

It is necessary with float to find out which is the tin side and which is the air side. The tin layer of the glass produces a bloom that resembles devitrification when compressed. Put the tin side down for a slump.  If you slump with the tin side up, you will create a tin bloom by compressing the tin. If the tin is on the bottom, you will be stretching the tin and so avoid the tin bloom.

Sharp, pointed and raised corners are the result of devitrification.  Devitrification is the crystallisation of glass. Mild devitrification appears to be dirty streaks across the surface. Extreme devitrification produces a crumbling glass surface. Raised, sharp corners are the result of intermediate devitrification. The tin side does not protect against devitrification.  It does provide a separating action when against the shelf, although kiln wash is still needed.  Float glass devitrifies easily. I have only ever been able to get two firings without devitrification.

Cleaning is of great importance in avoiding devitrification. Clean well with only a little detergent, rinse and then polish dry with paper towels. Any residues left on the surface will promote devitrification.

A general way of reducing sharp corners is to nip or round the corners with diamond pads. I nip the corners - it is quicker and does not leave any microscopic pits for devitrification formation.

Paint, stains and enamels will interact with the tin to produce variants of the colours.  Stains most often become darker than when put on the air side. Powder, frit and mica will not usually react to the tin.

Remember, float glass is not manufactured to be a kiln forming glass.  You will always be at risk of devitrification.

Cleaning Glass Before Painting

If your glass is not really clean, you can get gaps in the paint line. You may also have areas where the paint beads up rather than flows evenly.

You need to clean as best as possible first. Then just before painting you can use a dilute solution of the paint to scrub all over the glass with your finger or other firm material. Wipe any residue off with a paper towel and you will find that the prepared paint will flow evenly onto the glass.

Fusing with Painting

Combining Painting and Fusing

Painted oyster catchers with frit and stringer

It is possible to combine glass painting with fusing. Tracing paints are generally powdered iron rust and fused to the glass by the glass powder that carries this pigment. So it is possible to paint and fuse a project at the same time without loosing the intensity of the paint.

In general it is best to work down from the highest to the lowest temperature in your firings. This does require planning of the firing sequence in addition to the usual design considerations.

This sequence of firing depends on the glass stainers' paint you are using. The tracing paints (blacks, browns, some whites, some blue greens) that fire at 650ºC and above can be fired up to around 800ºC without losing much of their intensity. If you use Debitus paints, they can be fired to 850ºC without loosing their depth of colour.

Fused, painted and slumped piece painted both at fusing and slumping operations

If the paint is under glass pieces or under frit, the paint will appear to spread and the lines thicken. This is due to both the lens effect of the covering glass and the weight of the glass over the lines. If you require the lines to be of consistent thickness, you probably should paint after fusing.

You can, of course, use low firing ceramic glazes as they mature in the region of 700ºC to 850ºC. These can be painted on to the unfired glass and taken to full fuse without any fading. You do need to make sure the glaze has time for any volatile materials to burn off, so a slow rate of advance up to the slumping temperature of the glass is advisable.

Painted and fused, then painted and slumped.  Note the paint lines and coloured glass do not always match or need to.

If you are using glass stainers' enamels, you need to fuse and shape before firing. You can fire in the mould for the enamel firing as the temperature range is in the 520ºC to 580ºC range and will not add more mould marks to the glass. Keeping the glass in the mould protects against any tendency for the glass to alter shape.

Matting

Oil, and Water and Gum as Media for Matting by Dick Millard [edited from a discussion]

Oil has been used, I believe, since the 16th Century, and certainly up through the 1970's to today. It is used wherever it is determined it should be used, and one is sufficiently informed and facile to use it in a manner of delivering its full and lovely potential.

First of all, oil is not characteristically employed as a matt, out of which, by the negative process, one "takes out lights". In overwhelming instances, with which I am acquainted, it is used as a shading material applied over a pre applied and "worked" under matt of water and gum base.

This provides the required "tooth" to provide both a degree of adherence and ease of application.

So, I would suggest an oil matting, or a shading application over a smooth glass surface, would be generally problematical!

A group of blending brushes

Add a bit more gum to your water under matt which will reduce the necessity to fire that matt, which changes the character of the desired "tooth". The purpose of the "tooth" to receive the oil matt is also to provide "porosity" as an "absorbant", which additionally holds the oil mixed paint to the matt. Otherwise, the oil remains too liquid and does not float in a controlled fashion. It will require a much dryer application of kerosene, or increased absorption by additional blending.

I had a large landscape piece, hills in the back ground, that I matted and applied an alcohol mat too, but I was lifting the water mat trying to cover it with alcohol, so I added more gum to my mat and that did the trick. I also used a very soft Chinese brush. I have found that firing the mat first and looses tooth.

A group of stippling brushes

I have noticed over time that some people seem to have the impression that the less gum used, the better. I advise not to use an excessive amount of gum arabic, as a soft matt, with a soft touch produces a soft look. This is interpreted to mean 'less is better'. That is true, but only up to a point. If too little gum is used, or none, it will come off as if it were flour or mud diluted with water and applied. Too little gum severely jeopardizes any opportunity to produce soft gradation from the highlight to the untouched matt.

Antiquing Sandblasted Glass

Sometimes a sandblasted area appears too white when finished. One method that can be used to tone down the whiteness is to use low temperature glass stainers' enamel.

The low temperature enamels cure at temperatures between 530C and 580C depending on the type and manufacturer. At this temperature the glass is unlikely to change its shape. The jewellers and ceramics enamels fire at higher temperatures and are not suitable.

Rub the dry powder into the sandblasted area with a cloth or your fingers. The advantage of using the powder dry is that it will not stick to the smooth areas, although you may need to brush it out of any depressions in fused glass.

Fire the glass to the minimum temperature for the enamel, but for S96 or Bullseye try to stay below 540C. This temperature will fix the paint to the glass, but not change the shape of the sandblasting. Float glass will not change if you go to 580C. If you go to higher temperatures, you will go toward a satin effect and finally a smooth surface.

This technique has the advantage of being able to introduce a subtle colour tone to the sandblasted area. This enables you to match older glass that may have a slight colour cast from the glass or materials it has become encrusted with, such as nicotine.

This method requires testing to get the right levels of colour, and the temperature to balance the fixing of the enamel without changing the sandblasted surface beyond your choice. So you need to prepare several samples noting the amounts of enamel and temperatures used.

This has been successful for me when replacing broken sandblasted door panels that need to match the side lights. It removes the excessive whiteness of the new panel and can blend to match the colour of the originals.

Air Brushing on Glass

Raphael Schnepf Workshop

Air brushing onto glass is a little different than onto other slightly absorbent surfaces. As glass cannot absorb the moisture from the material being sprayed, the medium needs to be allowed evaporate. This means that each layer of paint must be allowed to dry before the next layer is applied. If too much liquid is applied to the glass, it will bead up giving a stippled appearance to the finished result.

There are some things that can help to give an even application of the paint or enamels to the glass.

Clean the glass very well. After thorough cleaning and drying, use some of the paint to rub the glass. As the paint is a slight abrasive, it cleans off anything the other cleaning methods could not get off.

Add a drop of washing up liquid to the mixture of paint and medium (liquid). This breaks the surface tension of the medium and reduces the tendency to bead up on the glass.

Use alcohol part or all of the medium. This reduces the evaporation time. Also apply in a warm rather than cold place. You can use a hair dryer on low speed and power to assist the drying.

Apply in thin even layers, allowing the paint to dry between applications.

Open the air brush trigger before reaching the edge of the area to be painted and close it after reaching the other edge. Any overspray can be cleaned up as in any other painting.

A slightly larger opening at the nozzle is required on the air brush than for other paints, but you have to be careful to avoid opening it so large that you get the spitting of large drops of paint onto your surface.

Because you are putting very small particles into the air you need to observe various precautions. You need to have a dust mask on at all times you are air brushing. You should do this in a spray booth with extraction if possible. If not, you need a well-ventilated area and very good clean up afterwards.

Paint – Temperature Effects

This is based on Graham Stone’s work with float glass. The temperatures are applicable to float glass, and so need to be adjusted for other glasses, usually a bit lower. But these temperatures illustrate the principle of how heating temperatures affect the paints. The temperatures will need to be adjusted when fired on other glasses than float. Temperatures are given in degrees Celsius.

570 Low firing glass enamels fired
650 Silver stain fired.
690 Low fire red enamel burnout.
730 "Paradise" paints fired.
750 Onglazes fired.
800 Lustre burnout begins.

Based on Firing Schedules for Glass; the Kiln Companion, by Graham Stone, Melbourne, 2000, ISBN 0-646-39733-8, p24

Media for Glass Enamels and Paints

Mixing agents
These are the carriers that give "tooth" to the paints and are water-based or oil-based.

Common water-based media are:
· water & gum arabic,
· wine,
· sugar water,
· vinegar

Common oil-based media are:
· clove oil,
· lavender oil,
· damar varnish

Gum arabic
This natural gum (also called gum acacia) is a substance that is taken from two sub-Saharan species of the acacia tree, Acacia senegal and Acacia seyal. It is used primarily in the food industry as a stabliser, but has had more varied uses in the past, including viscosity control in inks. For artists it is the traditional binder used in watercolour paint. It is sold in powder and liquid forms.

Dammar gum
This is obtained from the Dipterocarpaceae family of trees in India and East Asia, principally those of the genera Shorea, Balanocarpus, or Hopea. Most dammar gum is produced by tapping trees, however some is collected in fossilised form from the ground. The gum varies in colour from clear to pale yellow, while the fossilised form is grey-brown. It is used in foods, as a glazing agent, and in the making of incense, varnishing and in other processes. Dammar was first introduced as a picture varnish in 1826 and is commonly referred to as Damar varnish.

Painting Tracing Lines on Glass

Testing the thickness of the paint
Using your smaller brush, load the paint into it, and practice applying black lines on a clear piece of scrap glass. If the paint seems too thick, add a very small amount of water.

Paint that does not stick
If the paint seems to bubble up or not adhere to portions of the glass, it is likely the glass is not clean. You can wash the glass, or simply add a little more water to the paint already on the glass and rub the paint over the glass with your finger or a small piece of paper towel. This will remove any dirt or film of oils on the glass.

Testing the amount of gum arabic
Allow your painted lines to completely dry. You will notice the dry paint has a chalky, opaque quality. Test your paint lines by rubbing a finger across a line. If it easily rubs off like powder, you need a bit more gum arabic. If you can't budge it and it seems hard and crusty, you have much too much gum in the mixture. Adjust the mixtures accordingly.

Inspecting the fired glass
When the glass is fired in the kiln, the paint (which is made of ground glass and various ground pigments) fuses with the glass. Too much gum in the mix, and the paint may bubble, sit on the surface, or do a few other ugly and unprofessional tricks to embarrass you. You have no choice but to start over with a new piece of glass. Getting the amount of gum arable right is crucial to the process.

If the lines are not consistent in colour depth, you can trace over them and fire again. This will darken the lines to a consistent level of colour.

Practice
Practice your tracing. This part of the art is like calligraphy - half the battle is learning to use your tool, the tracing brush, in one or two confident strokes. The quality of your trace line tells the world whether you are an amateur or an accomplished artist! You might even decide this is the only glass painting technique you will ever use. And you would be in good company. A good deal of Gothic stained glass relied solely on tracery for its embellishment and to good effect.

Glass painting tools

The tools needed for glass painting are few and relatively common, although the blender is specialised. The minimum you need are:

Glass palette –
a lightly etched glass sheet on which to grind and mix the paints

Palette knife –
a paint knife with a flexible metal blade used to mix and pile the paint

Tracing brush –
a thin and long-haired brush used to apply paint to glass. Sable is considered superior as it can hold a lot of paint allowing long lines.

Badger blender –
a wide and flat brush made of badger hair used to blend or evenly disperse a layer of paint on the glass, or to stipple a fine layer for a pin-hole effect

Stippler –
a round, thick brush used to apply wet paint and create a stippled matte.

Enamels

In a discussion of art technology, enamel (or vitreous enamel, or porcelain enamel in American English) is the colourful result of fusion of metals carried in powdered glass to a substrate through the process of firing, usually between 750C and 850C. The powder melts and flows to harden as a smooth, durable vitreous coating on metal, glass or ceramic. It is often applied in a paste form and may be transparent or opaque when fired. Vitreous enamel can be applied to most metals.

Vitreous enamel has many excellent properties: it is smooth, hard, chemically resistant, durable, can take on long-lasting, brilliant colours, and cannot burn. Disadvantages are its tendency to crack or shatter when the substrate is stressed or bent.

Low firing enamels formulated for glass are forms of paint designed to fire between 550C and 600C which avoids the distortion of the glass that would occur with enamels designed for metals.

Preparing the Glass Paints

Paint
Measure about one tablespoon of black paint powder onto the center of your palette, With the edge of the palette knife, break up any clumps in the pigments. Grind the paint with the flat side of the palette knife until it feels and sounds smooth. Then mound the prepared powdered paint into a pile. Using the blade of the knife, pat the paint flat to about 1/8" thickness.

Gum Arabic
Then using the end of the palette knife, take some powdered gum arabic and, gently tapping the knife, sprinkle the gum over the surface of the paint. Use only enough to give the impression of a faint dusting of snow or sprinkling of salt. This step is crucial to the end result, as too much gum arabic makes removal of the paint from the glass with brushes and sticks difficult. Too much gum arabic can also cause the paint to bubble and splatter at firing temperatures. The correct amount may require some trial and error, but it is better to have less than more gum arabic.

Mixing
Blend the gum and paint together while dry. When thoroughly mixed, push into a mound, and make a valley in the center. Add about 1/2 teaspoon of water and with your palette knife, blend the pigment with the water. Add water and continue to blend until the paint is silky smooth and the consistency of thin yoghurt. Work the paint with a circular motion across the palette, then repeatedly "pile" the paint to the center of the palette.

Glass painting Media

Mixing agents
These are the carriers that give "tooth" to the paints and are water-based or oil-based.

Common water-based media are:

• water & gum arabic,
• wine,
• sugar water,
• vinegar

Common oil-based media are:

• clove oil,
• lavender oil,
• damar varnish

Binders

Gum arabic
This natural gum (also called gum acacia) is a substance that is taken from two sub-Sharan species of the acacia tree, Acacia senegal and Acacua seyal. It is used primarily in the food industry as a stabiliser, but has had more varied uses in the past, including viscosity control in inks. For artists it is the traditional binder used in watercolour paint. It is sold in powder and liquid forms.

Dammar gum
This is obtained from the Dipterocarpaceae family of trees in India and East Asia, principally those of the genera Shorea, Balanocarpus, or Hopea. Most dammar gum is produced by tapping trees, however some is collected in fossilised form from the ground. The gum varies in colour from clear to pale yellow, while the fossilised form is grey-brown. It is used in foods, as a glazing agent, in the making of incense, varnishing and in other processes. Dammar was first introduced as a picture varnish in 1826 and is commonly referred to as Damar varnish