Wednesday, 27 April 2016

Scum on Ground Edges

Almost without exception, ground edges show scum after fusing.  This scummy appearance is devitrification. This is caused by the powdered glass from grinding remaining in the pits caused by the action of refining the shape of the glass with a grinder. 

The suggestion that the glass should be placed in water immediately is of course a good precaution, although addition of vinegar is less efficacious than grinder lubricant added to the soak water.  This lubricant helps to keep the glass in suspension rather than settling into the scratches and pits of the grinding marks. The vinegar, which is often recommended, does nothing other than smell up the place.

The glass needs to be made smoother than the standard grinding bit will achieve.  Normally, a 600 grit grinding bit will be sufficient to allow a good fire polish without any devitrification. Sometimes 400 grit will be enough. You will need to step down in grit from the standard (about 100) to fine (about 200) to at least super fine (about 400) grit.  If you can find a 600 grit bit, that can be your final smoothing before cleaning and placing on your piece for fusing.  Of course, this grinding can be done by hand with wet and dry sandpaper without any great labour.

There is, of course, a more simple solution - don't grind. I rarely grind any pieces for kiln forming.  Often, this is because I am working thicker than 6mm and know the gaps will fill during the forming.  If I need to make adjustments for 6mm pieces, and I often do, I groze the edges of the glass.  This gives a much cleaner break of the glass than grinding.  Of course, the edges are not as precise as when ground, but the glass remains absent of all the scratches that harbour the devitrification.  Often the fit does not need to be precise anyway. 

When the fit does need to be precise, the parts that do not fit perfectly can be filled with the appropriate colour of powder. This should be kept as near the gap as possible and piled up only a little over the gap to compensate for the lack of mass that powder has in comparison with sheet glass.  This powder technique, of course, does not work well on tack fused pieces.  There, the grinding and smoothing needs to be pursued.