Wednesday, 1 February 2017

Devitrification on Ground Edges

The first element in preventing devitrification is cleaning.  Making sure all the edges of the glass are clean will help.  OK, you have cleaned the edges well after grinding. You still get detrification, so you want to know

Why do ground edges get devitrification? 

To answer this question, you need to think about how glass behaves in the kiln. As it heats up the glass expands, pushing the cut edges into the separator on the shelf. The pits caused by the grinding have not yet become fire polished.

When the glass retreats on cooling the pits in the edges of the glass, although very small, pick up some of the separator. These small particles act as the nucleation points for the crystallisation of the glass which is generally called devitrification.

The glass of a single 3mm layer retreats further on a single piece than on a 6mm piece. This rolls the devitrified glass upward onto the upper edge of the piece.

Prevention of devitrification of the ground edge is to have the pits in the glass edge finer than the particles of the separator. This is more than just washing the glass immediately after grinding to remove the glass powder from the grinding scratches.  Yes, this will reduce the chance for devitrification, but not totally prevent it.  As noted above, the pits in the glass will pick up particles of separator on expansion, giving nucleation points for the devitrification.

Further coldworking beyond the initial grinding is required to reduce the devitrification possibilities.  This involves using finer grinding bits or smoothing by hand with finer grits.  This does not have to take long, as the shape has been achieved by the grinder.

The logic of prevention is to have the glass edge smoother than the particle size of the separator, so the finer and smoother the separator, the smoother the surface of the glass edge must be.  

But my devitrified edge was on top of other glass

The follow-on question is about why devitrification occurs on ground edges that are not near the kiln shelf.  There are two elements to consider.

It is claimed that the fumes of the binder burning off can settle in the pits of the ground glass, providing those nucleation points for the glass crystalisation. The suggested solution is to vent the kiln to about 400C to allow the combustion fumes out of the kiln rather than keeping them inside the kiln.

The second and more certain element is that the grinding creates microscopic pits and fractures in the glass where the powder from grinding settles.  Almost no amount of cleaning will completely remove this residue from the tiny pits and fractures resulting from grinding. 

There are at least two solutions to this cleaning problem. Don't grind unless absolutely necessary - groze instead.  The second is to lightly cover any ground edges with clear powder frit.  You could of course consider ultrasonic cleaning or power washing, either with a dishwasher, or outdoor power washer.  Both these seem to be so completely out of proportion to the problem, that I have never used them.

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