Glass painting is a kind of drawing painted on the inside surface of transparent glass with gum or paints for the viewers to appreciate from outside the glass.

Archeological discoveries prove that China had been able to make glass by the Western Zhou Dynasty (1100-771BC). Glass painting, originated in Europe, was introduced into China by Italian painter Giuseppe Castiglione (1688-1766) in the Qing Dynasty (1644-1911), and it soon became popular in cities such as Beijing, Tianjin, Shanghai and Guangzhou, etc.

At the first beginning, glass painting was employed to decorate palace buildings, and later it developed into craftworks such as screens and palace lamps. During the reign of Emperor Qianlong, glass painting spread among ordinary people, and was much favored by wealthy people in China's coastal areas.

Glass painting is a relatively new medium, where proprietary liquid paints are used to color glass. Usually an outline is applied and is filled with the color. Pale tints are possible and colors can be mixed to obtain colors not possible with the original stained glass, only they are less durable and should be protected from the elements.

The subjects for glass painting are extensive, ranging from landscapes, historical resorts, birds, beasts, fishes and worms, to exotic flowers and glasses. There are literally dozens of ways to paint on glass, ranging from using traditional oil paints to using specialized glass paints that require firing with a kiln.

Traditional stained glass painting

Traditionally, glass painting referred to painting on the surface of a sheet of glass to be included in a stained glass work. This kind of painting, which is actually closer to drawing than painting, was done to add details such as faces and folds of clothing that couldn't be added with traditional lead lines. It was also used to cover up portions of stained glass works so that light was kept from shining through.

In most cases, the glass paints used for stained glass painting are predominately browns and gray-blacks. The colors tend to be water or gum arabic based, and can be applied with a brush in a method similar to the way watercolors are applied. In most cases, these paints are fired onto the glass using a kiln. The heat of the kiln causes them to bond permanently with the glass.

There are several major types of traditional stained glass paints, including vinegar trace paint, matt paint, silver stain, and oil-based paints.

** Vinegar trace paint

This paint, which is dark and completely blocks out the light in the areas where it is applied, is most often used for figure or design lines. It is fairly thick and must be mixed with water, vinegar, and gum arabic to use. Gum arabic, which helps the paint stick to the glass, is usually purchased in powder form and must be mixed with water or alcohol before using.

Vinegar trace paint must be applied "wet on wet"; that is, both the brush and the glass surface must be wet. You can't apply more paint to a particular place once it dries; if you do, the paint is likely to flake when fired in the kiln.

Painting with vinegar trace paint requires practice. The hardest part is learning to apply just the right amount of paint. Too much on the brush and it will blot, too little and it will dry before the stroke is complete.

When dry, vinegar trace paint is often scraped or scratched with a small stick or quill. This gives the paint a texture and depth that can't be gotten from the paint alone. Once prepared, the paint is fired to around 1100 degrees Fahrenheit. It becomes shiny after firing.

** Matt paint

Matt paint, which uses a base of either water and gum arabic or water and vinegar, is easier to apply than vinegar trace paint. It can be applied thickly or thinly and can even be "blended" and stippled or worked with a second brush to give it an interesting texture. Some artists even rub it with their fingers to achieve more unusual effects.

Because it is more transparent than vinegar trace paint, matte paint is generally applied over tracing paint. Often, two firings are required, one for the tracing paint and a second for the matt paint.

Matt paint is most frequently used for filling in backgrounds and adding shadows. As with vinegar trace paints, the color selection is somewhat limited, consisting primarily of blacks, brown, blues, and greens.

** Silver stain

Silver stain, which is available in shades of red, yellow, and orange, gets its name from the presence of silver nitrate in the stain. After firing, it turns golden, not silver-colored. It is unlike paint in that it actually changes the color of the glass, rather than simply covering it up with a dark line or wash.

Silver stains do not flow well from the brush, but since they are generally used to add accent colors (rather than detailed lines) this is not a major issue. They are often applied to the opposite side of the glass from vinegar trace and matt paints, and may be fired face down, with the silver stain resting on the kiln shelf.

Since silver stains are fired to around 1000 to 1100 degrees Fahrenheit, they may be fired at the same time as stained glass paints. Unlike glass paints, silver stains darken and grow deeper with each firing.

** Oil-based stained glass paints

The advantages of oil-based glass paints are that they come in more colors, are easier to work with, and are not effected by general atmospheric conditions. The major disadvantage of these paints are that they tend to be less consistent in application; although colors may be mixed like regular oil paints, they do not always mix easily or thoroughly and sometimes fire unevenly.

Oil-based paints, which use an oil-turpentine base, are generally fired to a slightly lower temperature than water-based paints. They tend to break up if fired to higher temperatures.

Paints made for surfaces other than glass

It is possible to paint on glass with paints that are made for painting on other surfaces. All you need are some paint and a desire to experiment. Oil paints and acrylic paints can work, as will widely available alternatives such as model paint or automobile paint.

These kinds of paints can be brushed on by hand, but airbrushing frequently yields better results. The advantages of this approach to painting on glass are that the paints are easily found, relatively inexpensive, and can be mixed into virtually any color. In addition, no kiln is required, although firing in a kiln can help oil paints dry and adhere better.

The disadvantage is that since a kiln is not used to permanently fuse the paint to the glass, paints made for other surfaces have a tendency to peel or chip when used on glass.

To counteract this tendency and increase the life span of the paint, you should concentrate on thoroughly preparing the surface of the surface of the glass. Sandblast if at all possible. At a minimum, rough the surface up with wet/dry sandpaper. Roughing the surface in this manner will help the paint adhere better and last longer. Applying the paint with an airbrush, rather than by hand, also tends to yield better results.

Air-dried and oven-cured glass paints

In this approach, the painter starts with a clear sheet of glass, then colors the glass in a pattern that approximates "real" stained glass. Sometimes practitioners of this style even go so far as to apply paste outlines designed to resemble stained glass lead or solder lines. In general, people who make stained glass frown upon this style of glass painting.

These paints do, however, have the advantage of being specially formulated for painting on glass. They come in two varieties: air-dried and oven cured.

As the names suggest, air-dried glass paints are simply applied and then allowed to dry (drying takes about eight hours). These paints are most suitable for decorative purposes on surfaces that will not be handled much. In most cases they can withstand only light washing in cold water, and are not suitable for food-bearing surfaces or items that will be handled.

Oven cured paints are a bit more durable. They require the use of a conventional kitchen oven to heat the glass to around 300 to 400 degrees Fahrenheit. This is hot enough to help bond the paint to the glass and will adhere better than glass that isn't heat-set at all, but the temperature isn't hot enough to fuse permanently.

Glass enamels

Although traditional painting on stained glass is probably the most common way of painting on glass, painting with glass enamels has grown significantly in popularity in the last few years. The range of colors is extensive, and the ability to mix colors means that a virtually unlimited palette is available.

These "paints," which like traditional stained glass paints are actually finely ground glass particles with a relatively low melting point, are available in both transparent and opaque colors. They are applied, and then fired onto the glass using a kiln.

Because glass enamels are made of tiny glass particles, care must be taken to ensure that they are compatible with the base glass being painted on. As with any other glass, using incompatible enamel will result in cracking or poor adhesion.

Care must also be taken when using glass enamels that the fine particles are not inhaled. A respirator or mask is recommended to prevent this, as inhaling fine glass particles can cause silicosis, a serious and potentially fatal lung condition.

There are a number of ways in which glass enamels may be applied, but in most cases the enamels are applied and fired in several layers. This maintains the integrity of the colors and also helps to achieve effects that would not be possible in a single firing. It is not uncommon for enamels to require four or five or more firings before the work is complete.

The most common ways to apply glass enamels are by brush, by dry sifting the enamels onto the glass, and by screen-printing.

** Brush application

To apply enamels with a brush, you must first mix the glass particles with a liquid (called a "medium") to obtain a paint-like consistency. Mediums may be oil or water-based, but water-based mediums have the advantage of generally being safer and easier to clean up. The key criterion is that the medium fires clear without leaving a residue.

The proper consistency for painting is approximately one part enamel to two parts medium, but this can vary depending on the particular enamels used. It is a good idea to mix the paints on a smooth surface (a sheet of window glass is ideal), adding the water drop by drop until the desired consistency is reached. Sometimes the paint will dry out a bit during painting and you will need to add a few extra drops of water.

Any brush may be used, but recognize that transparent enamels tend to show brush marks. Take advantage of this by using the brush to create desirable patterns and textures. (Opaque enamels are less likely to exhibit this trait.)

Once application is complete, the enamels should be allowed to dry prior to being kiln-fired. Enamels will mature at various temperatures, with opaques generally requiring higher temperatures (around 1450 degrees Fahrenheit) and transparents needing only to be fired to around 1200 degrees Fahrenheit. For enamels that fire at this lower temperature, it is possible to both slump and fire on enamels in the same firing. Many of these lower-firing enamels contain lead, so care should be taken not to use them on food-bearing surfaces.

It is possible to achieve significantly different results by varying the method of brush application. The enamels can be mixed very thin and splattered onto the glass. They can be applied, allowed to dry, then scratched partially off to yield interesting patterns. Often, the best results come from very thin applications and multiple firings; thicker applications tend to result in a dark, muddy appearance.

Also, many glass artists use "reverse painting" techniques. This approach, which is contrary to "normal" painting techniques which start with the background and add details as a final step, starts on the backside of the sheet of glass. Working from the top layers to the bottom, successive layers of paint are added and fired until the picture is complete. Although the approach appears strange at first, it results in a work with an exceptionally clear and glossy surface.

Remember that when firing enamels you must follow the normal warm glass procedures to heat, anneal, and cool the glass to prevent cracking and thermal shock. Fired properly, the enamels will bond to the glass, resulting in a permanent, lustrous finish.

** Dry application

Glass enamels can also be applied dry. To do this, simply place a small amount of enamel powder in a sifter and sift. You can add interest by drawing patterns in the sifted powder or masking off a portion of the glass "canvas" to control where the powder goes.

If you work with glass enamels, it's a good idea to wear a mask to keep from inhaling the glass particles. This is especially good advice if you are sifting and working with dry enamels, which can easily become airborne.