Wednesday, 11 July 2018

Adding Colour to Slumped Pieces

Sometimes an already fused and slumped clear piece needs colour for interest, definition, etc. The problem is how to do it without altering the fused piece.

You can use cold paints. Both acrylic and oven baked paints can be applied, but often they are unsubtle, harsh colours.  These are not permanent.

You can apply enamels.  These can be the ones produced for glass fusing, if fired carefully. The curing temperature of 780°C means that you need to fire slowly to about 600°C and then quickly to 780°C with no soak and AFAP to annealing.  This fast rate of advance is to preserve the shape as much as possible at temperatures above that required for slumping. This will need to be done in the mould, of course.

You can more safely use traditional glass stainer colours, which are also called enamels, although they are slightly different from the traditional ones.  These cure between 520°C and 580°C so can be fired as normal in one steady climb to the top temperature with no soak and quickly down to annealing. To be sure the shape is retained, the glass should be in the mould during the firing.

Use of frits and powders requires the higher temperatures that will distort the piece unless fired in the mould. When firing to tack fuse in a mould, you need to be careful that you do not damage the mould during the higher temperature firing, nor get the separator incorporated into the powder.  If you can place the powder or frit on top of the glass, you will get a better result at a lower temperature as the frit will heat and spread more easily on top than on the bottom. 

In general, it is not recommended to add colour to slumped pieces with frits and powders.  It is hard on the mould, and risks the glass sticking to the mould. Even if successful, the slumping mould will have to have the existing kiln wash removed and new added to avoid the kiln wash sticking to the next piece to be fired.  

It is better to slump the piece to flat, if possible, and then add the frits and powders before fusing.  Then slump again.

These notes show that it is important to assess the flat piece critically before proceeding to the slump.  This can mean setting the piece aside for a few days to review your impression of the fusing result.  This little time elapse can give you a fresh view of what the piece requires, if anything. 

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