Book: Ceramics - Art or Science? Author: Dr. Stan Jones

11. Pottery Technology 2

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Silk-screen printing could also be used to apply the paint, where a stencil is made that could be fairly crude, such as by cutting a pattern in strong paper or more accurately and finely by a photographic process. The stencil is then supported on a fine silk or wire mesh screen, and the paint is squeezed, using a rubber blade, through the stencil and silk onto paper.

Screen printing diagram where A is the paint, B rubber blade, C stencil, D screen, E frame and F pattern on transfer paper - source Wikipedia via Harry Wad. Screen printing equipment - source Flickr via com bertogg

Screen printing diagram where A is the
paint, B rubber blade, C stencil,
D screen, E frame and F pattern on
transfer paper - source Wikipedia via
Harry Wad. Screen printing equipment
- source Flickr via com bertogg

Each colour is put on individually and dried. The paint and paper is then covered by a clear coating, the “covercoat”, originally made of shellac, but now a plastic medium. This is robust and acts as a carrier making it easier to handle the transfer. The paper is soaked off the back of the transfer, leaving the colour and covercoat, which is applied to the vessel surface using a sponge or bat. The transfer is applied wet so it can still be moved within reason, which is a significant benefit. The subsequent firing would burn off the coating and any other non-oxide colour materials. Today, lithographic suppliers take designs and produce multi-coloured transfers on large paper-backed sheets having the plastic cover coating together with a layer of soluble gum. They are cut up by the operatives, soaked in water and the transfer slid off the backing paper onto the pot. Being flexible it can be used on non-flat surfaces.

Silk-screening can be used to apply painted decoration directly on to a pot. Using a small screen with the pot rotating beneath, a single colour can be applied. It is particularly used for single colour business promotions such as mugs.

The latest method is by direct digital printing, using a development of the laser printer and very finely milled powdered oxides.

11.10 Gilding

As mentioned earlier, gold is often used in decorating, a process known as gilding.

Gilder at work - courtesy Worcester Porcelain Museum

Gilder at work - courtesy Worcester
Porcelain Museum

The only metals that can be used directly for decoration are gold and platinum, as all other metals oxidise on firing. Gilding was little used on earthenware as it was very difficult to make it stick, but it was easier with porcelain and bone china. Gold can be used in two forms, one, “bright” gold, is applied as a soluble compound in an oily liquid producing a thin (0.1 micron) layer. The other is “burnished” gold, using mixtures of powdered gold or mercury amalgam, which forms a thicker layer that is dull after firing but shiny after burnishing. To produce burnished gold, the gold, usually 22 carat, is powdered, mixed with a liquid medium, painted on in one or two layers, possibly using a coloured undercoat of green or brown, and then fired at 560 to 740 degrees C (probably the fourth firing on a painted item).

Also mentioned before, platinum is used for decoration as it retains its natural colour, whereas silver does not. In the early 19th century Wedgwood used bright gold and platinum by powdering them and dissolving them in Aqua Regia (a mixture of nitric and hydrochloric acids). Then, in the case of gold, it was mixed with turpentine, linseed oil and flowers of sulphur and applied to the pot for firing in a reduction muffle kiln.

Today polishing or burnishing of the initially dull gold is carried out in two stages, firstly using a wet cloth and silver sand and secondly using a dry chamois with silver sand to achieve a high shine. Stone burnishing, typically using agate, gives a chased effect on the gold.

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