Breaking through the grey matter

The cast went into the kiln at 3pm on Thursday 4 October and now, 10 days later on Saturday 13th October, it's time to find out the results.

On first inspection the red glass hasn't moved from its saucers.  It has melted as expected however it would seem the exit hole was too small for the glass to seep through.  The surface has a crazed cracking as a result of the difference in the glass to terracotta cooling coefficient causing compression in the glass.  The glass surface is also heavily devitrified making it look opaque even though tests confirm this is a transparent glass.

In contrast, the large pots have discharged all of their glass.  An inspection of the pots shows that there has been some colour mixing inside the pot.

Looking at the cast for the first time I can see that the colours have mixed really well and that the turquoise blue has created some very deep colour tones.  On closer examination I can see that the mould has not been filled with glass.  There isn't very much left in the pots, just the usually traces expected, and there isn't much in terms of over spill from the locations the glass poured down.  I have to conclude that the weight used was insufficient despite adding an extra 100g to compensate for the residue and any overspill.  This could have resulted from weighing the clay after it had dried out a little and therefore shrunk to a lesser volume.

I considered leaving everything in place and adding the shortfall in glass to the pots and re-firing however I had to be satisfied that the mould could withstand another firing.  I noticed that there were some cracks in the plaster which needed further examination.  There were two cracks that clearly moved from the upper surface, down the side and underneath.  I could have reinforced the mould by binding it with high fire wire however when I inspected the cracks it transpired they all met underneath and the likelihood was that the mould would split if re-fired.  This proved to be the deciding factor and I decided that I had only lost less than 10mm and that I could work with this.

I'd no sooner started to scrape out the plaster form the top plug when the side cracks started to move, boy was I glad I chose not to re-fire!

I didn't want to cause any unnecessary stress in the glass so I took my time scraping break lines in the plaster and freeing it off in small sections.  It didn't take too long to free the glass entirely and after a quick brush down and wash I could see how well the glass had picked out the details of the mould, particularly when compared to the silicone rubber mould taken from the same plaster mould.

The detail below shows the difference in the glass finish from the areas that have been in contact with the plaster from those that have been free from contact, at the top of the mould.  This detail also shows a shoulder that has been created from the overspill of the glass pouring down onto the mould.  I will a lot of cold working both on the finished surface and also on the underside as this has to be completely flat and smooth to ensure a good bonding with its carrier glass for the final display.