Gilding upon glass is done in the same manner as described for water gilding, isinglass size being used in the place of water. The glass is well cleaned, freed from grease, and set before the operator at a slight angle, sometimes the glass is upright, as in a window, and has to be done in that position. The isinglass size, before described, is used in precisely the same way as the water in water gilding, and the gold laid on the flowing size so as to stretch itself out as the size recedes. The size must be used freely and allowed to run off quickly. It must not be strong, the weaker it is the brighter will the finish of the gold be. The less size there is remaining between the glass and the gold and between the two coats of gold, the better polish can be obtained.

In all other methods of gilding the gold is attached from the back of the leaf, and the finished work shows the unalloyed brightness of the metal, but in the case of glass gilding, the size comes between the gold and the eye, and the glass interposes a further medium, so that it is at once apparent that the cleaner the glass, and the clearer and thinner the film of size, the less is the brilliance of the gilding interfered with. The purity and cleanliness of the size and glass will be assured if the size can be laid upon the glass without cissing or gathering. If it runs off like water on a duck's back, the glass is greasy or the size is not clean, or perhaps the water used is too hard, boiled rain water makes the best size, but it must be clean and clear.

Gilding on glass requires a second coat in order to make a solid job. The first coat of gold when dry is lightly polished with finest cotton wool, and fixed and burnished by scalding with very hot water as near boiling as can be used without splitting the glass. It may be poured over from the spout of a kettle, so as to run over the whole of the gilding, and then down on to the ground, or laid over with a broad 4-inch camel-hair flat. This removes the scum of the size from between the gilding and the glass and adds to its clarity and brilliance. The work may then be carefully polished with a piece of finest cotton wool. It is then allowed to dry and the whole of the gilding and clearing with hot water repeated. After this the gold is backed up by a coat of hard Japan or varnish which will dry in about eight hours and have a perfect gloss. In cold weather the whole of the glass must be treated with the hot water whether gilded or not, or breakage will result from the inequality of expansion produced, and if the day be frosty, the job must be done very cautiously in a hot shop, or deferred.

The water must never be boiling.

The gold used for glass gilding is specially prepared, being more even in thickness than the ordinary gold, and put up in books of special paper that does not require dusting with French chalk or Armenian bole to prevent the gold adhering to the book. The gold thus supplied is much cleaner than that used for general purposes. It is important that glass gilding be made to dry off quickly and that no time be allowed to elapse between the operations, or it will accumulate dust and get discolored.

Although gilding on glass is looked upon as a difficult matter to successfully carry through, all the difficulties are overcome by the exercise of cleanliness. The cleanliness of the glass may be tested by breathing on it, and if the moisture evaporates quickly, leaving the glass clear, it will do. Glass may be made chemically clean by the use of dilute nitric acid, and well rinsing with water.

Tissue paper is a good glass polisher. Filtered rainwater makes the best isinglass size, or distilled water, as it is free from metallic taint.

Gilding upon paper, parchment, and vellum can be best done by using a size made from yolk of eggs and glycerine. This is ground together with a little ochre and thinned with water. If used in a very liquid state as a mere water wash size, and the gold is laid directly thereon, as in glass gilding, it may be tooled or burnished. All gold work should be sized before writing or painting upon it.