Frosting on slumped glass

 [We’re] Having a few challenges with a stainless-steel S-curve mould (15cm x 10cm [prepared with boron nitride]. … When we slump a piece of glass, we get a frosted effect … in places where the glass was touching the mould at the beginning. I don't think it's devitrification, because the glass itself isn't cloudy, it's just hundreds of little bumps and dimples.

Photo credit: Adrian Cresswell

 This is a mould that combines draping and slumping in the same firing. The glass must drape over the hump and slump into the valley at the same time. This is effectively two processes in the same firing. It does require some compromise in scheduling as a result.

 The evidence presented shows boron nitride – a slippery surface – was used to prepare the mould. In another firing Thinfire – a powdered surface – was used as a separator. Both created this marking on the back. The schedule was not presented.

 This indicates something other than the separator is creating the problem. Note that the marking also occurs at the extreme right end where the glass would be resting on the lip of the mould. The marking does not occur where the glass is slumping down into the curve. It only occurs where the glass is draping.

 As suggested, this is not devitrification. That occurs on the surface rather than on the bottom. This further indicates the difficulty is between the glass and the mould.

 I suggest the marks are from the glass sliding along the mould. These are frequently called stretch marks. The glass is sliding and stretching along the mould. This blog post contains much more information.

 It is of course possible that insufficient boron nitride was placed on the steel and the glass grabbed the steel. It is worth checking, although I don’t think it likely.

 You might think the Thinfire covering of the steel would make everything smooth. However, Thinfire turns to powder and fibreglass particles after about 400°C/753°F. These particles are drawn along as the glass moves against the mould. The particles can bunch and remain as bumps on the surface of the mould. This may account for the rougher surface with Thinfire than boron nitride.

A summary

These stretch marks occur when the glass moves excessively against the mould. This is usually a combination of high temperature and fast ramp rates. Slumping should be done at the lowest practical temperature. The soak should be long, rather than brief.

The Remedy

 Fire more slowly and to a lower temperature. The Bullseye suggestion from their quick tips is for a double curve (or wave) mould of 250 x 210 x 40 mm (9.85" x 8.25" x 1.6"). They suggest a ramp rate of 167°C to 660°C /1221°F with a soak of 10 minutes for a 6mm/0.25” thick piece.

 In my experience this is too fast. Slumping into this mould can be done at 630°C/1167°F with a 30-minute soak in my kilns using the same ramp rate.

 This is a simple mould to slump and drape into. It is essentially two partial cylinders pushed together in opposite directions. The curves are gentle and progressive. There are no sharp changes of direction. These factors mean that the slow and low approach will work well.

 However, in this case the curves will be even tighter as the full length is 150mm x 100mm/ 6” x 4”. The height is not given, but for a self-standing piece, it likely to be a minimum of 40mm/1.6". This makes for a tight curve on the mould. It is likely the glass will slide more than on a gentle curve. My thought is that the steel mould has been produced without knowledge - or testing - of the practicalities of getting the glass to bend to such a small radius. This means that I would be trying a schedule of about 125°C/225°F per hour to a top temperature of about 630°C/1167°F with a 1.0 to a 1.5-hour soak. The glass is going slump much more slowly with this smaller span.

 With gentle heating -slow ramp rate, long soak - the glass gradually conforms to the shape of the mould without stretching over the hump/crest of the mould. Instead, what happens is that the glass slips slightly from the opposite end of the mould. To counteract this, I place the glass 6mm/0.25” over the upraised end of the ceramic mould. This then finishes just inside the mould’s edge.

 With a steel mould, this is not possible without the glass hanging up on the hot and sharp edge. The glass will need to be at the edge or just inside to prevent hanging up on the end of the mould. The glass will slip down the mould a little, but not so much as to cause problems. It is possible to prop a piece of fibre board at the upturned end of the wave mould to support the overhanging glass if the full curve is required.

 The glass on the high part of the mould will not stretch at the low temperature, but gradually conform to the shape of the mould.

 I have much more information on this and other things in the eBook:

Low Temperature Kilnforming; an Evidence-Based Approach to Scheduling