Achieving the Striking Colour

"Is there anything special I have to do to fire striker glass?  Can I mix striker and non-striker in the same kiln or piece?"

Strikers generally need a two-hour soak at slumping temperatures, about 660C.  This heat soak helps ensure full development of the colour. If the soak is not long enough, the colour may not achieve the target colour at all, or be paler than anticipated.

The rate of advance to the heat soak is not critical.  But it does need to be the appropriate rate for the thickness and nature of the assembly of glass being fired.

If you were to have too short a heat soak, you can fire again to help mature underdeveloped colours.  This will, of course, change the profile of the finished piece.

Strikers are compatible within their manufacturer’s own range. So, they can be combined in the same piece as any other of the glass in the fusing compatible range.  That means that they can be fired in the same kiln load as non strikers.

The two-hour soak at slumping temperature will not harm the later stages of firing, but it might lead to use of a slightly lower temperature tack fusing than without the long heat soak.  That is because of the heat work put into the glass at the lower temperature.   Only observation will tell you how much less temperature is required.  It may be possible that only a little less time at the forming temperature is required.  Again, only observation will tell you that.

Strikers require a heat soak to mature the final colour.  These striking glasses are compatible with the rest of the fusing range from a single manufacturer. Glass from different manufacturers must be tested for compatibility before combined into a project.

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.