The Art of Mistakes August 27, 2026 18:54

There's a side of glass work that you have to spend a lot of time on, and it's not the pretty finished part. It's testing - spending days, sometimes weeks, getting things wrong. Colours don't fire the colour you expected. Heat whirls or updrafts move a critical element – imagine that thin sliver of glass you placed ‘just right’ ends up blown halfway across the panel. Then imagine waiting 24 hours just to find out the blue you added is actually the wrong shade of blue, and without testing, you can only tell once it's been through the kiln.

It sounds like it should be simple. Pick a colour, put it on the glass, fire it, see what you get. In practice, almost nothing behaves the way you'd expect from looking at it in the jar or the image on the computer screen. A powder that looks vivid unfired can disappear entirely on a dark base. A line drawn confidently in glass paint can vanish completely in the kiln, burnt away to nothing. A glass stringer (thin piece of glass like a thread) might hold its shape perfectly but have shifted several centimetres across the base to a different part of the piece. You won’t know until it's been fired.

What I'm Testing at the Moment

This week, I was testing two things – in between writing a CNZ application and trying to finish work for 2 different exhibitions. 

First, how a colour reads against a background – in particular, against an opaque black background, which is unforgiving in a different way to clear or pale glass. A glass powder I was given looks warm and orange in the jar but the jar is labelled red (I know right!!) – an obvious need for testing!  But it also might be a completely differently shade once it's dusted onto black and fired. 

Something else worth explaining here is that temperature also dictates how much some colours will 'strike'. Striker glass looks completely different before firing to how it ends up afterwards - a pale pink or yellowish sheet, unfired, could transform into a rich red or bright orange once it's been through the kiln. That transformation comes from heat causing chemical changes in the metallic compounds inside the glass, things like cadmium or gold, which activate and turn a dull or colourless material into its target hue. How far that colour develops depends heavily on the firing schedule: the top temperature and how long it's held there. Too little heat-work and the colour might be underdeveloped, producing a lighter colour change. Too much, and it can degrade or shift the tone entirely.

The only way to know is trial and error.  Test it at different temperatures, on different bases – if you have time.  Or you have to test it on an exact background, at the exact temperature you need for the project. For example:  I might only be firing a single sheet of 3mm glass, which means I can't go to as high a temperature as I would normally, because of the 6mm rule - *** have I shared that one yet? It's worth repeating, because I call it the 'golden rule'.

Glass loves to be 6mm thick, which is handy when you're working with sheets of glass, since they come in 3mm thickness. Cut two sheets to the same shape, lay them on top of each other, fire a standard schedule, and you won't get any movement - they'll fuse together but generally keep the shape you cut. If you used 4mm glass and fired the same schedule, it would move at top temperature, pulling in to try to make itself 6mm. With 8mm of glass, it will try to spread out to become 6mm instead. The golden rule: glass loves 6mm!

When I'm only firing 3mm - a single sheet - I need to change my schedule to a lower temperature to stop it pulling in towards 6mm. But, that lower temperature may impact the colour strike of the glass. Tricky, right?  Insert lots of eye rolls here lol.

The second thing I'm testing at the moment is a collection of non-glass additions I've gathered over the years - odd bits and pieces advertised as kiln-compatible, meant to leave a line, mark or bit of detail that survives the firing intact. This is where things get interesting - and occasionally frustrating. I've been testing everything from glass marking crayons and Glassline chalks through to metallic markers, checking which ones hold a crisp line and which ones blur, spread, or disappear entirely once heat gets involved. Some marks fire beautifully, holding their shape and colour almost exactly as drawn. Others move - a fine line becomes a vague smudge - or a mark that looked strong pre-fire doesn't survive at all.

The two test strips I've got here show a bit of both. One is a straightforward colour test - a series of glass powders fired side by side on black, so I can see exactly how each one reads once it's done its time in the kiln, rather than guessing from the jar or the image from the manufacture’s website. The other is a mark-making test, checking how a metallic paint and a couple of different crayons and chalks behave when fired onto black glass, including whether the line stays crisp, spreads, or disappears altogether.

   
The Even More Frustrating Bit

Here's the part that tests anyone's patience: you can't just peek and check. Every single test has to go through a full firing cycle - up to top temperature (in a series of prescribed heating and holding phases), held, and then cooled (also in a series of cooling and holding phases)all the way back down to room temperature before you can safely handle it and actually see what happened. Depending on the schedule, that's 24 hours, sometimes longer, before you can see your test strips.

So…. testing isn't a quick afternoon job. It's slow, deliberate, and more than a little bit of a waiting game. You load the kiln, close the lid, and then you wait - no shortcuts, no sneaky peaks halfway through, just patience until it's properly cool. It's tempting to want to rush it, especially when you're keen to know if something worked. But glass doesn't reward impatience. Open a kiln too early and you crack a piece, on top of not actually knowing your result yet anyway.

If it’s that hard, why do it?

Because it's the only way to really know - seeing a specific colour, on a specific base, fired to a specific schedule. Full stop.

Every test becomes part of a growing reference, one I can go to next time I'm planning a piece, so I know with confidence what something is actually going to look like once it comes out of the kiln, rather than hoping for the best.

It's not the glamorous side of glass art – but then I’m still looking for the glamorous side! It's the part that makes the finished pieces actually work.

I recently came across a quote by Toss Woollston, which I really love – I think it sums up glass artists – “the hope of success”.  Toss was talking of his own early art practice, but I honestly feel that this is what drives every glass artist – the hope of success!

Made by hand. Made to keep.