White Opaque Glaze Testing

I wanted to make a glaze that I’ve used in various capacities work better as I was having some problems with it.  This meant that I had to go back and figure out why. I got the glaze from Linda Arbuckle’s site. She is the queen of Maiolica. I went through my copy of Val Cushing’s handbook and realised ‘Bills Basic’ is the same glaze. So from now on I’ll refer to it as BBLA. Below you can see even a theoretical coefficient of expansion and unity formula.

vchandbook

This is the page of Bill’s Basic from Val Cushing’s handbook.

Suspension

As you can see from the recipe, there isn’t a lot of clay content.  This makes it hard for all of the other ingredients to stay in suspension.  I added 2% dry weight of bentonite clay.  This will keep primarily the frit and the other ingredients together.  NB mix the bentonite into the dry mix.  If the glaze is already wet, you can soak down the bentonite overnight with water.  It is very clumpy on its own so if you’re adding it later you’ll need to sieve it again.

Flocculation

Now the big problem I was having with this glaze was the coverage. I needed to gel, or flocculate it to stay where I put it, and not drip. You can do this easily and with control with distilled vinegar. A cap full or two into a 5 Kg batch should be enough. Now the glaze goes on evenly, and doesn’t drip.

Specific Gravity

I wanted to have a consistent thickness between batches, so I had to find the specific gravity. This is a lot easier than it sounds. You need a graduated cylinder to measure fluids in science. When the glaze is in the right consistency, tare a gram scale to the cylinder, measure 100ml and weigh it.  I found that around 160 or 1.6 gave me the best coverage. Now when mixing a new batch, you can add water to the glaze until you get the same gravity and it will be the same as the previous batch.

Hardener

The last thing was that I was smearing the glaze around when applying colour. I needed to introduce a hardener. I chose gum Arabic as I had some but there are others. I added 1% to the dry weight of glaze. This will dramatically slow the drying time down, but it will produce a hard surface where you can apply colour on top.

Conclusion

These and many other techniques can be used on all glazes. They will help get more consistent, repeatable, and favourable results. Below is my revised recipe for BBLA:

BBLA January 2016 revision: (fire to 1060°C or Δ04)

Ferro Frit 3124 65.8
Soda Feldspar 17.1
China Clay 10.8
Nepheline Syenite 6.3
Batch 100.00
Additions:
Zircopax 14
Rutile 0.5
Bentonite Clay 2
Gum Arabic 1
Vinegar 1-3ml