With crystalline, the glassworkers at La Rochère use a multitude of colours in powder, grain and bar form to design the various items.
La Rochère can also colour one of the pots in order to obtain a homogenous colour (such as amber or sky blue), to produce a romantic service, for example.
With machine glass, La Rochère does not use a great deal of colour, but when it does, it is mass-colouring. This involves adding oxides in frit form (flakes) to the composition and colouring the glass throughout. This makes the colour dishwasher proof - an essential feature of daily use.
Prior to moulding (being blown in the mould), the glass, which is still at 900°C, is tempered in water, dusted with potassium carbonate and then covered with a second layer of glass. The effect of the heat makes the potassium break up, freeing carbonic gas which is then trapped in the glass, forming bubbles.
The item (still at 900°C) is abruptly submerged in water during the moulding cycle. The thermal shock created crackles the glass and the residual heat ensures cohesion.
Multi-layer sandblasted glass
Multi-layer sandblasted glass involves the overlaying of several layers of different colour.
Whilst hot, the operation consists of moulding the article by blowing and turning in a cast-iron or wooden mould. The glassworker applies several different-coloured layers of enamel before reheating the item. A taker-in places the item in an annealing furnace, called a tunnel lehr. This operation removes the thermal constraints to which the glass is subject during working, or brings them down to acceptable levels, to prevent it from breaking in the event of a mechanical or thermal shock. In the case of some lamps, the annealing cycle can last for over five hours.
A lehr assistant inspects each item as it comes out of the tunnel.
The work continues after cooling, when stencils (rather like stickers) are placed on the item.
The items are then sandblasted, with the pressurised sand attacking the parts left visible by the stencil. The expertise of operators is of utmost importance during this stage, as they have to precisely judge the shades of the design.
A thread of liquid gold is applied with a thin brush on the rim of the glass and it is annealed at around 550°C.
Glass is cut by abrasion, using various types of diamond grinding wheels. The brightness of the cut is obtained by polishing with cork and pumice wheels, followed by further cerium oxide polishing with felt wheels. This mechanical process has the advantage, compared to acid polishing, of not smoothing the cuts.