And a video demonstration
57 new subscribers this week – it’s great so many people are willing to work hard improving their glass painting techniques.
Make no mistake: working with us is often tough, but you’ll get your own reward.
And I don’t just mean the free video at the end of this post.
I mean: your own achievements.
Today – blistered paint – you know, when glass paint bubbles and cracks in the heat of the kiln: 5 causes and 1 myth.
But first, here’s why you must know how to do it properly …
Thick, dark paint – its many uses
For the stained glass painter, its distinctive use is to “frame” a painted image – to “block in” that image and thus intensify the effect of transmitted light:
As you saw last week, the right-hand gargoyle is far more effective than the gargoyle on the left. The thick, dark paint makes everything more dramatic.
But thick, dark paint is often also used within an image (not just around it), as in the gargoyle’s eyes and mouth, and also in this heraldic crest … of which you’ll see a useful video in a while:
Now flooding is a lovely, simple technique, so there are plenty of reasons for you to get it right.
Also, many ways it can go wrong.
Let’s get going.
Remember the problem we’re examining is why thick dark glass paint sometimes cracks and blisters in the kiln.
And since there are so many new arrivals this week, I’ll spend a moment debunking the myth which your long-standing colleagues know all about.
- The myth is: you cannot paint on top of unfired glass paint because if you do, the paint will always blister.
You know, I’ve got at least 10 books which say this, and it’s just not true.
Newcomers – check out Part 1 of Glass Painting Techniques & Secrets from an English Stained Glass Studio (free chapter here), and you’ll see why this view is wrong: one of the first things you learn with us is how (when you want) you can do several layers of tracing and shading in a just one firing.
And that’s right: no blistering.
See, the problem isn’t caused by painting several layers. It’s caused by other things.
First the big causes. Second, the video demonstration. Third, the firing schedule.
So let’s now make a start.
Cause #1 – wrong paint
David and I have tested many different brands of glass paint. Our clear preference is for Reusche.
A good all-purpose mix is made from tracing black (DE401) with umber brown (DE402) in rough proportion 4:1 – it’s wonderful for all consistencies of glass paint from undercoats right through to silhouettes.
And yes, even we‘ve had blistering problems with different paints than Reusche’s.
Note for newcomers:
- We’re independent.
- We’re not paid sponsership money or advertising fees.
We tell you things because they work, that’s all. And Reusche paint is excellent.
Cause #2 – too much gum Arabic
With too much gum Arabic, your paint is almost bound to crack: it will just bubble and blister in the heat of the kiln.
So what’s the “right amount” of gum?
There isn’t a scientific answer here. Conditions vary. But here’s a useful and all-purpose test:
- Prepare a batch of glass paint.
- From this batch, prepare a small quantity of undercoating paint.
- On the light box, paint a swatch of undercoat, blend, and allow to dry.
- Repeat, so you have a double-layer.
- Make sure your hands are clean and dry. Gently rub the undercoat with the soft part below your thumb.
If it’s very difficult to bruise this double-layer of undercoat, this tells you there is too much gum Arabic in the paint. If you use this paint for flooding / silhouetting, it is likely it will blister in the kiln.
On the other hand, if the undercoat is easily and immediately rubbed away, this tells you there might be too little gum Arabic in the mix. You may wish to add more.
If the undercoat comes away in a measured and controllable way, that’s when we’d be happy with it.
Cause #3 – badly mixed reservoir of paint
Now what’s the “reservoir”, you ask?
You start with a lump of paint. (Not a teaspoonful – that’d be wasteful. See Part 1 section 1 for why.)
With your palette knife, you cut away some slices, add some water to get the right consistency for flooding, then mix it all together.
This is your “reservoir” – your pool of working paint.
Now the thing about glass paint, gum Arabic and water is … they’re not best friends! The water evaporates. The gum splits off from the water. And the silica and oxides in the glass paint just sink to the bottom.
That’s why, each time you load your brush, you must re-mix the paint.
If you don’t, the gum in particular will not be evenly distributed throughout the paint.
And that means some parts will have a higher density of gum than others.
And this is a very common cause of blistering.
The only way to prevent it is: re-mix your paint each time you load your brush. As you’ll see in the video, it only takes a few twirls.
Cause #4 – bad painting technique
Images are better than words here, so I’ll keep it brief because you’ll see it in the video.
Here’s what happens.
The moment the paint leaves your brush, it starts to dry and set.
This means you just have a few seconds to push the paint where you want it. No more than that. Then you must leave it to find its own level.
See, if you move the paint once it starts to dry, you’ll undo your good work in getting the paint well-mixed in the first place. Which again means some parts will have more gum than others. And they’ll crack and blister.
Now watch what I mean.
Yes, it always helps to watch.
And unless you’re lucky enough to have a friendly master glass painter at your call, you won’t ever see close-up scenes like these ones here.
So turn on the volume and watch closely.
And note the “full-screen” icon on the right …
I hope this helps. As always, questions welcome.
Now for the fifth and final cause, which I can’t show you in the film.
Cause #5 – wrong firing schedule
This is easy: if you fire flooded paint too quickly – meaning, it doesn’t have proper time to dry out – you’ll give your glass every encouragement to blister.
- So this is an easy thing to put right: just adjust your firing schedule.
- Make it slower overall. Rise more slowly to your first ramp. Stay there longer. Rise more slowly to your top temperature.
Here’s what works for us. We always start by taking one hour to move to 100 Celsius / 210 Fahrenheit, where we soak the glass for 15 minutes. Then we go to top temperature over three hours.
I hope this helps.
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