A lot of glass-works are the result of the ability of the craftsman who successively embellishes the surfaces with so-called cold techniques. In many cases, the ability to decorate transforms simple vitreous objects in real works of art.
Let's look at some of them:
Engraving consists in making shallow lines with a pointed tool which is harder than glass (vidiam* steel, corundum, diamond) on the surface. It is a very ancient technique which was applied to glass as a derivation from hard stones.
The groove looks opaque so drawings or designs or simply writing on glass can be traced. In addition to hard pointed tools, other tools are used in engraving, even very small wheels or rotating pointed tools attached to a drill similar to a dentist's. Many maestros and glasswork companies use this technique to sign their objects.
Engraving (or cutting) on the wheel, on the other hand, consists of removing surface glass, by means of abrasive powder applied to wheels which turn at high speed. The result is like that of bas-relief-type work where the cut indents create figures, decorations etc..
The process involves three successive operations: the actual cutting itself using with bigger-grained abrasives which creates the rough object, then an intermediate stage with finer abrasives, and the final polishing (where required) with cerium oxide applied to soft supports like cork, plastics or felt cloths.
This last stage makes the surface perfectly shiny by removing the opaqueness formed from the previous stages.
Corrosion of the surface of glass is obtained by immersing the glass in a mixture of hydrofluoric and sulphuric acid. The real corrosive is hydrofluoric acid, the sulphuric acid is required to dissolve the insoluble fluorosilicates which form and which would tend to deposit themselves on the surface.
The parts of the object which must not come into contact with the acid are covered with impermeable material (waxes or fatty materials) so that the acid acts only on the exposed areas. Depending on the concentration and relationship of the two acids, the temperature and contact time of the glass with the acid bath, either deep or surface, shiny or opaque acid cuttings are obtained.
The surface of glass can be attached and thus sandblasted by a jet-stream of sand sprayed by compressed air against the surface.
The fineness of the frosting depends on the type and strength of the jet-stream and the grain-size of the sand. It is a more economical and ecological method than acid cutting. The parts of the surface which do not require frosting are protected by metal masks which are cut with the designs that need to be reproduced on the glass.
There are three techniques for painting on glass.
1 - The traditional glassworks technique which uses coloured sheets: a brush is used to mark our outlines and shading; grisaille is then used, which, for furnace firing, is irreversibly attached to the glass.
2 - Painting with meltable coloured enamels, either transparent or opaque, which are fired to melt it into place.
3- Cold painting with colours which are set in place by the drying of the binder and without heating. Though this can be easily removed by abrasion.
This is glass with a low melting point, which means that is contains a lot of low-temperature-fusion-oxides like the lead, boron and bismuth ones. They are coloured with the same oxides and compounds used for glass, but in much higher concentrations, given the fine thickness with which they are applied. They can be both opaque and transparent.
Painting colours are thus made up of a glass that has a low melting point which is needed to melt the colour to the surface of the object and a colouring agent.
The combination of the two components in very fine powder, carried in an oily organic binding agent, is applied on to the glass with a brush or jet-stream, left to dry and then heated in a muffle furnace at a relatively low temperature (about 500oC) to avoid distorting the object, up to the fusion of the flux.
Meltable colours cannot be treated in the same way as common colours: mixing them up does not always produce an intermediate colour.
Red colours have a gold base, yellow have a silver base, blues are produced with cobalt compounds, greens with copper and chrome.
Enamel-decorated objects are very delicate. Enamels have the disadvantage of being easily attachable even by acids and to break off from the supporting material due to the different thermal expansion.
These are a partiuclar type of enamel. Using these, very fine films - shiny or polished, non-oxidisable metals like gold, silver or platinum - can be put on to glass.
This technique uses yellow gold (pure gold) or white gold (platinum) coating of blown objects like the edges of glasses.
With this technique, the decoration is fixed using a plate placed on the surface of the glass. To do this, a metal frame is used on which a synthetic fabric is stretched (originally it was silk which explains the name of the technique).
The plate is obtained by a photo-mechanical system which opens the stitches in the parts of the drawing which have to be reproduced.
Glass with a low melting point, powdered and mixed with a pigment, is put on the frame and allowed to pass through the opened stitches of the fabric with a rubber spatula.
After drying, the glass is placed in a muffle furnace at the required temperature to melt the enamel.
