The colouring of glass with enamel

by Lucio Malvezzo

Seigneurs, I beg you consider carefully this treatise on the new science of colouring glass with enamel. I sincerely hope it pleases you sufficiently to consider allowing me to present future treatises to yourselves from the position a membership of this august body. The new process that I outline allows for glass to be painted like a picture on an easel and dispenses with the practice of using copious amounts of leading to join different colour pieces of glass to form pictures from a mosaic of coloured glass. I take no personal credit for the creation or discovery of this process however I believe I am the first to record for your eminences these practices currently being developed and employed in Lyon in France. Should your eminences see fit to promulgate these practices our own craftsmen might keep in the forefront of modern science and technology in this field and we may adorn our churches more clearly to illustrate God's good works.

I set out below what has been the traditional method of colouring glass first using silver stain and then the improvements that are now possible to the process and the benefits they present.

Silver Stain

In the 14th century a new technique was developed for colouring the surface of glass. Silver sulphide or silver nitrate was ground and mixed with water to achieve a creamy consistency, and then the stain was applied to the back of the glass panel. Before firing, the stained area was brushed with a large, soft badger brush to spread the stain out evenly or move it into areas where a more concentrated colour was required. The firing temperature for stain was much lower than that for enamel, ranging between 540 and 560°C, and for this reason a panel having both types of colour should have the enamel fired first and the stain second. Unlike enamel, silver stain actually bonds with the glass during firing, and thus is much less vulnerable to deterioration. The colour produced by the early silver stains varied according to the composition of the glass to which the stain was applied, the amount of stain used arid the number of applications, but ranged only between light yellow and an orange-red. The best glass for taking silver stain (on which a single application produced a deep golden orange) was kelp, a soft glass whose alkali was derived from seaweed. Numerous applications of stain on this type of glass could produce an almost true red, and it has been used extensively for this purpose.

Silver stain was most often used for colouring hair, halos and decorative details in clothing, and it allowed for the use of two colours in a single pane of glass, thus eliminating the need for leading to surround the yellow colour. Panes of blue glass could also be stained to produce green. Where flashed glass was used, stain allowed for the combination of three colours in a single pane, and it proved particularly useful in creating detailed heraldic designs in which golden crowns onions stood on a crest of blue and white or red and white. Further line details could be done in black enamel. The use of silver staining reached its peak in the last century when the perpendicular style of architecture dictated the need for enormous church windows.

Coloured enamels or 'Grisaille'

We now see the development of a new range of enamels which are translucent and coloured by the addition of various metal oxides. These enamels add blue, green and purple to the existing palette of black and brown. The new technique consists of melting the powdered lead glass base with the metal oxide first, then grinding the resulting coloured glass to a powder and mixing it with a medium to create a paint. When the painted glass panel is fired, the enamel regained its translucency and fuses to the panel as a thin layer of coloured glass. This innovation will creates a movement away from the use of leading to divide segments of pot-coloured glass, with artists choosing instead to exploit the new enamel's painterly qualities. This will allow for a new mastery of realism, and I suspect that much painted glass will consist of large, clear panels treated much like easel paintings. The leading, originally an integral part of the design as it joined the separate pieces of coloured glass, will soon serve only to hold large squares or rectangles of clear painted glass together.

This technique, known as the 'grisaille' or enhanced fiery colour technique (appeared in the first half of the 16th century) makes it possible to avoid cutting out as many pieces of glass as there are colours, and resembles more nearly the drawing of an engraving, another iconographic medium in rapid expansion at this time.

Seigneurs, You may well now be aware that I am from Venice and my family tradition is one of glass making working. A golden age of Venetian glass is beginning and since the glass makers have been grouped together on Murano a period of great experimentation has begun. I hope to be able to produce "cristallo", a clear, transparent glass which is extremely easy to work in its molten state. Soon Venice will be exporting cups, bowls and dishes in this new glass to all parts of Europe. The production of coloured, opaque glass will also continue and a new, milky white glass, called "lattimo" has been discovered. This looks to prove to be an excellent background for enamel decoration and if you will permit my membership of the Collegio, I hope to write further on this subject and on the introduction of filigree glass, decorated with a pattern of crisscross white threads.