Wednesday, 19 December 2018

Striking glass

Yes, much glass is striking in its effect.  But the term is used in a technical sense to indicate the glass has not reached its intended colour without further firing.

A striking glass is one that changes to its true colour. Not one which takes up a different colour.  There seem to be differing ideas on how striking works, but it is an intentional process.

Several glasses coloured with copper or silver strike to Their final colour when heated.  It seems that copper when used to make red (rather than blue or green) can undergo a chemical change during the heating.  The copper oxide used is normally Cu2O.  When heated the copper and oxygen molecules can separate and form bonds with other molecules.  The rapid cooling that is used in glass prevents the copper and oxygen from combining in the Cu2O formation.  The extent of this dissociation determines the degree of colour change.  Thus, the colour is affected by the heat work given to the glass – assuming the starting proportions of materials are the same.  This can occur with some other colouring metals too.

Another form of striking is caused by the growth of crystals within the glass. In these cases, usually in silver bearing glass, the metals separate from the silica and form small crystalline structures which are also fixed by the rapid cooling required for glass.

There is another theory that the colour change is due to the orientation of the colouring molecules within the glass matrix.  The idea is that the molecules will change from the clearer state to the struck colour due to the orientation caused by reheating and cooling.

The actual process seems to be unknown in a definitive sense.  What is known is that temperature, a reducing or oxidising atmosphere, and heat work will vary the intensity of the strike in colour.  This means that where the project is especially sensitive, you must undertake experiments to help predict the colour that will be achieved with the conditions you choose to use.