Wednesday, 13 June 2018

Volume control

Glass has a surface tension (viscosity) that draws the glass toward 6-7 mm thick at kiln forming temperatures. 

To test this out, prepare three stacks of glass squares.  They all should be the same size.  Place them in a stack of one, a stack of two and the last of three squares.  Fire them to a full fuse.  Compare the sizes of the original to see the expanded size of the three-layer stack, the same size of the two-layer stack and the reduced footprint  and dog-boning of the single layer.

Glass in a single layer behaves differently from the thicker set-ups. When the glass is hot it begins thickening at the edges because the viscosity is drawing the glass both from the edge and from the centre.  This means the footprint of the glass is getting smaller. The result is needling, as the glass retreats leaving small threads where the glass was held in small imperfections in the separator’s surface. 

If you do not need a full fuse, you can reduce this needling effect by reducing the temperature and extending the soak.  This means that the glass does not expand on the heat up so much and the greater viscosity reduces the needling effect.

If you need a thick piece of a certain size, you need to dam the glass to overcome the tendency to expand.  With experience, you can get to know how much a three-layer (or more) set up will expand and cut the glass accordingly.  In this way, you can often do without dams, although there will be some thinning at the edges in relation the centre and a rounding of the corners.

An excellent document on volume control is the Bullseye Tech Note 5.  

Note that this 6mm rule applies at normal kilnforming temperatures.

At higher temperatures, the viscosity is less so the glass becomes thinner.  My experience has shown that at around 1200°C the glass will spread to about 0.5mm thickness.  This is just to point out there is an relationship between temperature and viscosity, and therefore thickness. This relationship between temperature and viscosity -  as the temperature rises, so the viscosity reduces - allows the glass to become thinner.  At normal kilnforming temperatures, the 6mm rule applies, at higher temperatures it does not.

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