I’ve been having some beef with the most recent batch of White Tekta. Until now, I’ve had no problems with it at all, but the stuff I got in my last order has been playing up. After I convinced myself it wasn’t me, it was definitely the glass, I decided to do some investigating.

White Tekta has the same formula as 0113 (normal white opal). The sheets I’ve had up until now have behaved exactly the same as 0113, with the bonus that there’s no pesky rolled edges (I hate rolled edges – they explode in the flame and are just UGH). When pulled thin in a cane, 0113 sometimes can look a bit translucent, but it often opalises when the chip is full-fused in the kiln. I also make sure to make layers of white a bit thicker, and not to put it directly on top of a darker colour, to make sure the final fused murrine looks nice and white.

Therefore, it was a little surprising when I made up a cane in a recent batch of wintry (that one up there ^) and the cold chips were bright white! Not only that, it was a pain in the ass to pull, as well – 0113 is normally pretty well behaved. As I said, layers of white are never normally bright white when cold, they’re kind of translucent-ish. Immediately my spidey-sense started telling me something was off with the white. But to make sure it wasn’t just me doing something squiffy with the glass, I made another one and it was exactly the same.

A chip of spring green opal of the same mass as my average murrine chip, fully fused onto 6mm tekta – see below for more info. on method. Note the slight stress haloes.

The murrine chips came out from the testing run on the higher-end of what I consider ‘compatible’. That is, they are fine, they are still compatible and they will not cause any problems. They show no bigger stress haloes than a same mass chip of spring green sheet glass fused in the same way. But because I can’t control the firing schedules every single customer uses with my murrine, I prefer to have my chips showing much lower stress haloes than this (there are always small stress haloes because the chip has had more heatwork than the base glass, but these are minimal). As I’ve talked about before, increasing saturation of opalescent glasses often correlates with decreasing compatibility when tested. Brighter white = less compatible. It’s why I never use 0013 to make canes, except as stringers (very occasionally). And also probably why Bullseye issue a warning with (0313) Dense White not to give it too much heatwork. For some reason I always read ‘Dense White’ as ‘Denise White’. I have no idea why this is. I don’t even know anybody called Denise.

note the translucent edges on the 0113 (top 6) compared to the white tekta (bottom 4)

I was convinced it was the White Tekta. When you compare the cold sheets – the old batch had the same clear-bits at the top and bottom as 0113 (the pic is strips of sheet which I use in the torch on their sides). I know for a fact that a lot of the opals do this weird semi-transparent thing when cooled quickly, so previously I never thought much of it. Anyway, I decided to do a simple test for compatibility/sensitivity to heatwork that I do every day, I call it the lazy fuser’s stress testing method:


Step 1:
Cut a small square of the glass to be tested.

Step 2:
Weigh it and note this down. Mass can affect stress haloes a bit I have found, so make sure to always use identically sized squares if you’re comparing batches so that it’s a fair test (remember year 7 science?). Mine are 0.6g (+/- 0.05g)

Step 3:
Put the chip on top of two 2.5 x 2.5cm stacked squares of clear tekta. For bonus laziness points, use 6mm tetka.

Step 4:
Put it in for a full fuse*.

Step 5:
When the kiln’s done, take the chip and sandwich it between two pieces of polarising film and take a picture for reference.

*Because these chips are small, I use a fast (400degC/hr) second ramp schedule for them with a 30min bubble squeeze at 670degC. Heatwork is proportional to size, because it is faster for heat to even out through something small than something large. I always anneal my testing chips for 1h30 (so that I know the stress is from the glass, not the firing), and ramp down at 70degC per hour or slower for the anneal cool.

Stress haloes of identically sized chips of 0113 and White Tekta after being fully fused onto 6mm tekta base glass

As you can see, after the initial full fuse, the Tekta did have larger stress haloes than the 0113 – vindication is sweet. However, quite often I find that the more densely saturated opals become increasingly harder to work in the flame the longer they are exposed to the heat; they become stiffer with prolonged heatwork (the pinks do this, I hate them). So, to test this, I put the same two chips in for two subsequent full fuses. The results were quite surprising.

Stress haloes of identically sized chips of 0113 and White Tekta after a second full fuse.

The haloes on both glasses increased between fuse 1 and fuse 3, but the Tekta increased much faster, and by more. Which could be due to it becoming stiffer and more viscous with increasing heatwork, causing more stress. I have to mention here that the stress haloes on that bit of Tekta after three firings are nowhere near those from a chip of 96 on Tekta after only one firing (see pic). However, I’d certainly advise caution when exposing white tekta that looks opaque in section to repeated and prolonged heatwork.

Chips in cross-polarised light after three full fuses. Note the much higher stress haloes from the tekta on the left hand side.

I think the main thing to remember from this is not that white Tekta is ‘bad’ or that you shouldn’t use it. This set of experiments really should be taken as a reminder of the fact that glass is a very complex material, and it is not always a given that it will be exactly the same batch-to-batch. One of the manufacturers of 104 glass, Creation is Messy, have been trying to replicate a particular colour they made for years without success (it’s called Sakura, and it is beautiful. If any of you are lampworkers and have some… you lucky, lucky things!). It’s amazing that Bullseye have created sheet glass for us with such consistency despite this, but it shouldn’t be taken for granted. They remind us of this on their website – stating clearly that that their glass is handmade, and may vary slightly between production runs. Testing is easy and relatively cheap and it really helps to get a feel for this fabulous, complex artistic medium.

< The same thrice-fused chips as the picture above but this time the stress haloes are compared to those produced by a chip of system96 glass fused on top of 6mm tekta after a single firing.

Happy Fusing! — K