Air Brushing onto Glass

Air brushing paint and enamels onto glass can give extremely subtle graduations of colour and tone.

The consistency of the mix of the paint or enamel with the medium will need to be many times thinner than that used for painting with a brush. Also the air brush will need to be able to cope with the relatively large (in relation to inks and other paint) particles that make up the glass paint. This may require a little experimentation to find nozzle sizes that can cope with the glass paint particles.

There are two main media that you can use – there are others of course. Water and alcohol or methylated spirits are common and easily available. The advantage of spirits is that it evaporates from the surface more quickly. You do not need to use any gum arabic in the mix to help it flow. You could however add a touch of washing up liquid to overcome any surface tension within the mix.

The paint should be applied in steady sweeps across the area to be covered about 300mm from the surface. Start moving your brush before you switch on the paint and keep moving after you stop applying the paint. This avoids heavy applications at the start and finish. As there is no absorption of the moisture by the glass (as there would be on paper or board), you must pause after a very few passes. This is where the spirits show their value, as they evaporate more quickly than water, allowing you to apply the next layer sooner.

You can assist the drying by using a hair drier to gently blow warm air over the surface. This will also reduce the waiting time between applications.

One thing you will notice is that the paint will settle within the medium unless you agitate it frequently. So you should make sure the paint is evenly dispersed within the container by agitating it before starting each layer. The movement of the air brush during application will be enough to keep the paint suspended in the medium while you are applying the glass paint or enamels.

The relative order of kiln forming events

When preparing for multiple firings of elements onto a prepared piece, you need to consider the order and temperatures of events so that you do not harm an earlier stage of the project.  This blog entry will not give definitive temperatures, as that varies by glass and by kiln.  Instead, it indicates what happens in progression from highest to lowest temperatures in approximate Celsius degrees.  

ca. 1300C  -  Approximate liquid temperature 

ca. 850 – 1000C  -  Glass blowing working temperature

ca. 950C  -  Raking and combing

ca. 850C  -  Casting

ca. 810C  -  Full fuse

ca. 790C  -  Large bubble formation

ca. 770C  -  High tack, low contour fuse

ca. 760C  -  Tack fuse

ca. 750C  -  Fire polish

ca. 700C – 760C  -  Devitrification range

ca. 700C  -  Lamination tack

ca. 600C – 680C  -  Slump and drape

ca. 650C  -  Vitreous paint curing temperature

ca. 600C  -  No risk of thermal shock above this temperature 

ca. 540 – 580C  -  Glass stainers enamel curing temperature

ca. 520 – 550C  -  Silver stain firing temperature

ca. 550C  -  Glass surface beginning to soften

Slow rates of advance needed from room temperature to ca. 500C

These temperatures are of course, affected by the soak times. The longer the soak time, the lower temperature required. The rate at which you achieve the temperature also affects the effective temperature.  Slower rates of advance require lower temperatures, than fast rises in temperature.  These illustrate the effect of heat work.

The table shows for example you need to do all the flat operations and firings before slumping or draping.  It also shows you can use vitreous glass paints at the same time as slumping and draping.  This emphasises that the standard practice is to plan the kind of firings you will need for the piece and do them in the order of highest temperature first, lowest last.

In general, you do need to do the highest temperature operation first and lowest last.  But there are some things you can do with heat work.  For example, if you needed to sandblast a tack fused piece, but did not want to risk reducing the differences in height there things you can do.  From the list above, you can see the glass surface begins to soften around 500C.  It is possible to soak the glass for a long time around 500C to give it a fire polish, instead of going to a much higher temperature.  You will need to experiment to find the right combination of temperature and soak length, but it can be done.

This article is to show that knowledge of what is happening to the glass at different temperatures, can help in “fooling” the glass into giving you the results you want without always following the “rules”.  This may also be what it is to be a maverick glass worker.  Use the behaviour of glass to your advantage.

What Cartoon Lines Represent

A frequently asked question by novice glass workers is whether to score at one side of the line or in the middle.  This question revolves around the meaning of the cartoon lines.  What do the lines of a cartoon represent?

Meaning of Cartoon Lines

The lines on a cut line cartoon represent the space required between pieces of glass.  This will vary, depending on the style in which you are working.  In most glass working, a matrix of lead or foil is used.  The space required by these materials needs to be represented in the cut line cartoon. You may have other cartoons for other purposes – painting, came width, foil width, etc., but the lines in the cut line cartoon are there to represent the space required between pieces of glass.

An example of a cartoon for painting

Lead Came

In general, a 1.2mm line is required for standard lead came. This is close to the line made by a new bullet pointed felt tipped marker. If you are working with high heart cames, you will need a 2.8mm wide line. Some chisel point markers, if used on the sharp edge have this approximate width.

The glass is scored at the inside edge of the cartoon line.  This can be done by scoring directly on top of the cartoon, often with a light underneath.  You can make pattern pieces when the glass is too dense for enough light to come through.  If you must, you can draw the score line on the glass. You can score around pattern pieces, but if your scoring wheel goes over the pattern in any place, the scoring pressure will not be delivered to the glass.

Example of came varieties

Copper Foil

In copper foil, a much thinner line is used as the space between pieces of glass needs only be approximately 0.4mm. This is approximately the width of a sharpened pencil or ball point pen line.

The scoring is at the edge of the line as for lead came.  Also, you can score directly over the cartoon, draw on the glass, or make pattern pieces as for lead came projects.

Fusing Cartoons

When preparing a cartoon for fusing, the lines need to be as fine as possible.  The pieces of glass require no space, as they will be butted against each other.  However, unless cutting by computer controlled instruments, the cutting cannot be completely accurate, so the same size of line as for copper foil will do.

As you are going to try to butt the glass pieces together in fusing projects, you score along the middle of the cartoon lines.  As much as possible, cutting over the cartoon will give the best result.  Of course, there are many times when the light is not good enough and pattern pieces will be required. 

Another approach is also possible. Having scored and broken the first piece, you can place it on top of the glass to be cut for the adjoining one.  With a very fine felt tip or fountain pen, trace the edge of the first piece. Score down the middle of that line to create the best fitting second piece.  And so on through the whole project where the glass is not too dense to use a light box.

Conclusion

The line widths in a cartoon are determined by the space required between pieces by the assembly method.  The thicker the matrix material, the thicker the line and vice versa. 

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.

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.

Brushes for Painting

A quality paint brush will have hairs that form a point and have a good spring to them - they bend while painting but return quickly to their original shape. A good brush will also hold lots of paint and deliver that paint evenly throughout the stroke. Brushes usually have a number to indicate their size - the larger the number, the larger the paintbrush. The larger the brush the wider the line that can be produced, although with a light touch a fine long line can be made because of the pointed nature of the brush.

The best brushes are made from natural hairs, although there are brushes made from a combination of natural and synthetic materials which are adequate.

Sable hair brushes are considered to be the best for painting. The hair comes from a variety of pine martin and the Kolinsky sable from Siberia is considered the best. These brushes are more expensive than others, but are soft and flexible, hold their paint well and can make an expressive thick to thin line.

Ox hairs are normally used for making rigger brushes. This is a round brush with long hairs, said to be used to paint the lines of ships' rigging in the past. The hair is strong and springy making it useful for long lines and thicker paints.

Squirrel hair brushes are useful for applying paint in broad, thin layers for matting.

Goat hair brushes are normally known as hake brushes. These are a traditional, oriental style brush. It lacks spring, but forms a good point and so is useful to cover larger areas quickly with a gentle touch.

Pony hair is made into short round brushes used as soft stipplers.

Hog hairs are made into hard, very economical brushes. They come in flat and round shapes. They are most used for stippling and can be trimmed, shaped, used, and abused for years.

Badger hairs are thicker at the end and thinner at the root, creating a conical shape. These soft brushes are used to blend paint once it has been spread on the glass. The brush is swept across the surface of the paint to blend or move paint and remove stroke lines.

Cleaning Blending Brushes

Cleaning badger brushes just before use, is easy. Flick, gently and rapidly, the very ends of the brush hairs against the side of your hand – but use respiratory protection and be careful not to inhale any dust. If you notice flecks of dust in your paint when you create a grisaille you’ll know it’s time for a thorough and wet cleaning again.

After each use, rinse out the brush tips in cool water. Gently rub the tips of the brush hairs to loosen any extra paint. Grasp he hairs above the tips to keep the water from the main part of the brush. Then wet the exposed ends of the hairs and rub them gently until the water runs clear. 

If you use a blender for oil, you will need to use a small amount of natural soap, if so, thoroughly rinse.

Flick the brush to remove excess water, smooth the hairs into shape and allow to completely dry by hanging the brush with the hairs pointing downward – this avoids water flowing into the brush base where the hairs are attached. If you have round-handled brushes, you can twirl the brush between your hands to remove excess water.

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.

Fixing Paint for Transport

The very cheapest hair spray works well with glass paint, if you need to transfer your painted glass to another place for firing. Complete the painting and then spray with cheap hair spray as you would to fix a charcoal drawing. This will hold the paint firmly during transport and does not affect the paint during firing.

Tracing on Opalescent Glass

Opalescent and dense glass presents problems as the usual method of tracing the image through the glass is not possible. If you first spray the glass with a cheap hair spray, this gives a “toothed” surface to the glass. Then using carbon paper an image can be transferred. However, the carbon paper leaves a greasy residue, so water based paint will not take, but an oil medium will.

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.

Tracing with a Pen

Example of a pen nib

Using paint mixed with essential oil or turpentine and with a fine mapping pen for small lettering works well, as the oil flows better than water. Although with practice, a water based paint can be used with a pen, but it is a little tedious as the pen has to be loaded frequently with a tracing brush and constantly cleaned as the water dries quickly.

Examples of nibs and holders

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.

Light Box

Light boxes are in many ways a development from the glass easel. These were used in studios to wax up the painted glass and display it as it would be seen in a window. Sometimes the glass painters painted across all the glass at once, so this method enabled them to see the results immediately.

Nowadays people tend to use back lighting for these and other purposes, so the light box has become more popular. Some of the uses are outlined here:

The light box is very useful when tracing or altering designs. The back lighting enables you to use other paper than tracing papers to transfer the design elements. You can fold the paper along the lines of symmetry to check on how the lines match, or to copy the lines from one side onto the other side at the designing and cartoon stages.

The light box can help select glass colours either initially or when the main pieces have already been established. The combination of the glass over light shows how they interact with each other. At later stages when the main glass is cut, it can help avoid unwanted bright or dull areas.

Possibly the most common use is in cutting dark or opalescent glass. The additional light allows you to see the cartoon through the glass and so cut directly from the cartoon. This can be enhanced by blanking out the excess light from around the glass or cartoon.

The light box enables the arrangement of the cut glass pieces to be assembled to view the colour balance and have a virtual view of how the panel or window will look as a finished piece. An additional step toward the result is gained if each piece is outlined in white-board markers – use black pens - to represent the lead or copper foil and their widths. This stops the light between the pieces from causing you pupils to contract, and gives a more accurate representation of the appearance of the final pane.  White-board markers can just be wiped off the glass without using spirits.

The light box is important in painting. The back lighting shows the effect of the painted line or level of shading immediately. This allows adjustments to be made quickly and accurately before firing,

Depending on how the light box is built, it also can be used as display lighting. This can be as up-lighting or backlighting.

Additional notes:
Light box requirements
Flexibility
Top surfaces
Lighting

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

What is Waxing Up?

I have had the question above asked. It seems appropriate to respond as part of the tips section, as I had made unwarranted assumptions in posting the waxing up recipe.

Waxing up is the process whereby the cut and sometimes partially painted glass pieces are assembled on and stuck to a glass sheet - called a glass easel - before raising it to a window to get the light that it will receive when installed. This allows you to see what the current state of the window is and how it would look when installed. It shows up weak areas, or pieces which are not really compatible. Although it is used mainly by those who do a lot of painting on their glass, it is equally valuable to assess the composition of a leaded or copper foiled piece. It does ensure that you do not get surprises when you have finished a piece.

The wax used is sticky and stiff enough to hold the glass, but not so sticky as to be difficult to get the pieces off the glass or the wax off the pieces of glass being prepared.

Also some users of the glass easel method paint representations of the lead lines on the back side of the supporting glass to ensure the values of the lines are appropriate for the amount of detail for the various areas of the panel.

Waxing up Recipe

Beeswax alone isn't good for waxing up glass pieces on a glass easel, but combining it with Venice Turpentine makes a secure fixing material.

Recipe
250gm block of beeswax
2 teaspoons of Venice Turpentine

Method
Bring the two elements just to a simmer, stir it well and cool.

This produces a tacky material that when warmed in the hand sticks to the glass and the easel. When it cools it sticks firmly.

Use a palette knife to loosen the wax from the easel.

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 slightly 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 matt