Dog Boning in Slumps

I have done a few experiments on rectangular moulds with 3mm and 6mm thickness. I could not eliminate dog boning with larger rims, slower rates, or lower temperatures in any combination - although they did reduce the effect.

Square single layers dog boned even with increased rim width, and reduction of slumping depth made little difference in the amount of dog boning. 

Rectangular single layers shapes persisted in dog boning on the long side regardless of the rim dimension, and exhibited more dog boning on the long side than in the equivalent single layer square.  Two layer slumping had a decrease in dog boning with increased rim width, but with less effect on the long side.

In general, glass slumped in rectangular moulds is more sensitive the shape of the rectangle than the size of the rim, and very sensitive to symmetrical placing on the mould.  The depth of the mould has less influence than the size of the rim, especially for single layers.  The wider the rim, the less dog boning, in general terms.

Deeper moulds, higher temperatures, longer holds, narrower rims, all increased the dog boning. I conclude slumped square glass looks better because the dog boning is symmetrical.

My solution is to make bigger rims and cut the piece square after slumping. This approach needs cold work to the edges, of course.

The reason rectangular slumps dog bone is because the glass at the sides is drawn into the mould more easily than the corners, because there is more glass to draw in, just as in flat dog boning.

An alternative to the cold working is to round the corners of the rectangles to reduce the amount to draw-in.  A 1cm/0.375” radius curve will reduce the extent of the dog boning, but does not eliminate the effect.