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Posts Tagged ‘thickness’

There is nothing more disturbing than this.

Luckily it is unusual to wake up and discover that the laws of physics have been contravened during the night, but believe me, when it does happen, it puts a kink in your day, not to mention the universe. A serious kink not straightened out by the suspicion that the reason might be that the kiln is sick. Universe-bendingly sick.

OK, so what happened exactly? In your own words, ma’am. Take your time.

I went to the opera with a friend (Barber of Seville, since you ask, sung in English with the most fabulously witty libretto), came home, shoved some stuff in the kiln and went to bed. On the bottom was a red and white two-layer disc to go in the dog bowl mold, on top was a 30cm disc. The details of this are important. The bottom layer was Bullseye 2mm transparent glass in charcoal gray and dark plum covering approximately the surface area shown in the somewhat dodgy illustration to the right. Over it was a 30cm disc of 3mm Bullseye Tekta (clear). I set a top temperature of 1345°, for 15 minutes. I reckon the total thickness was 1½ x 3mm layers.

Now one of the few things I thought I really knew about glass was that when you heat it up to fluidity, it will look to be around 6mm (two standard layers) thick when left to its own devices. A thinner disc will tend to contract (to a footprint that will allow it to be 6mm thick), giving those characteristic needle-sharp projections where the glass is said to have “grabbed” the shelf (clinging on by its fingernails?) in contracting. A thicker disc will likewise tend to overflow its footprint until it reaches the 6mm level. Simple. I understand it to be something to do with surface tension.

I know this is why, when your toddler upends a glass of milk, it doesn’t just conveniently fall on the floor in an upside-down glass of milk shape but spreads out into an unholy mess over an area several times the size of the kitchen, covering all available surfaces (including, but not limited to, the dishwasher door, the walls and the ceiling). This is because spilt milk has a surface tension equal to parental tension/infinity -1. Mercury has a much greater surface tension, which is why a spilt thermometer will roll around forever like poisonous marbles. All it means to me in glassworld is that a dichroic pendant comes out a nice even 6mm even though it’s made of several different pieces of glass and would otherwise come out in lots of little lumps and bumps, or flow into a flat and fragile disc of immense thinness. So this much I thought I knew.

Why then, did the glass disc on the top shelf spread out to a thickness of about 2mm, such that it flowed right over the edge of the shelf on one side? And it split in half, to boot. As I say, worrying non-conformity to the laws of physics.

Except of course, it wasn’t that at all. Rather more mundanely, I think the kiln wasn’t level, because the floor isn’t entirely level (I’ve now pulled out the spirit level and am planning on checking obsessively before every firing). Also it almost certainly overheated because I may have had the shelf edge too close to the thermostat, baffling it in both senses of the word. The red and white bowl also came out not level, and needed to go back in. Now it’s fine: I’m fabulously pleased with it.

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I did a really stupid thing last week. I was cutting a 12″ disc using my new second-hand-from-eBay Silberschnitt circle cutter when a slight crack appeared running in from one edge. At the time I was pretty pleased with myself for getting the disc out neatly without it breaking across at the crack. I thought it would close up again in the kiln. Silly me. Probably that would have been the case if, a) I had been using two layers of 3mm glass rather than one, or b) I had thought to place part of the design over the crack, thus effectively providing the necessary double layer at that point.* As it was, with a firing to 1345° for 20 mintes, the crack opened up a couple of millimiters over a length of about two inches. Useless. And – by my standards – an outrageous waste of a square foot of glass. Not to mention that some of that glass was the ludicrously expensive sunset coral.

I have spent much of the last week mitigating this error. Otherwise known as throwing good time after bad.

First I thought about the possibilities of a) and b) above and decided to place another “leaf” over the troublespot where the crack had opened up. So the next night I did that. It didn’t work. The design could take the extra element, but the crack was still there, albeit smaller. A couple of days later I visited my friend and begged some coarse clear frit for repairs. According to Bullseye’s catalog, the coarser the grade of frit, the clearer it will come out of the kiln (I think they are talking about casting), so I figured I should give myself al the help I could get at this point, and the poor disc was already looking a bit sorry for itself and was risking devitrification with every additional journey into the heart of hotness…

Well, that sort of worked. At least there was no hole in it any more and there was some chance of turning it into a passable dish. Last night it went back in, in one of the new molds (the one they call the “round slumper”, which baffled me until I got the exact dimensions and then decided seems to mean “flattish dish”, but maybe they think that might look silly in a catalog).

OK. Not great. But OK. The mold is good, anyway. The dish is in the cellar, already.

*Put succinctly for the uninitiated, glass likes to be around 6mm thick. If you layer it thicker than that, it will tend to flow outwards at heat, if you use less thickness – i.e one layer of standard fusing glass – it will tend to contract as it tries to puddle into a 6mm-thick mass. Both these problems are minimized, if not eradicated, by a conservative firing schedule.

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