Borax Decahydrate (Na2B4O5(OH)4 8H2O) is a soluble source of boron.
Borax is not ideal for use in typical glazes because of its solubility.
| Temperature (C) | g/100mL |
|---|---|
| 0 | 2 |
| 20 | 3 |
| 100 | 60 |
These are approximate values for decahydrate. In other words, if you mix a glaze with 100g water for 100g dry materials, approximately 3g of borax will dissolve into the water.
It is possible to dissolve much more borax by heating the water first. When the water cools, the borax will stay in solution. Boron compounds (including Gerstley Borate) do seem to crystallize after going through freeze and thaw cycles, so glazes containing borax are best kept indoors.
| Temperature (C) | Description |
|---|---|
| 75 | Borax decahydrate decomposes |
| 743 | Anhydrous borax melts |
This table is incomplete. At 75C borax decahydrate decomposes, losing some of its water of crystallization, but not all of it. When does it decompose to anhydrous borax. Does it even do so at specific temperatures or does it happen gradually?
In his article Flambe Magic, John Britt finds mystery crystals growing in his glaze bucket after it was left outside in the winter. He identifies the crystals as borate compounds because the hydrates burned off around 315C and completely melted by 870C.
Borax is soluble in water, but only slightly. It would be amazing from a chemical perspective if boron could be used like a non-soluble glaze ingredient as it would provide an unbelievably concentrated source of both sodium and boron. After LOI, Borax is almost 70% B2O3, compared to only 23% for Ferro Frit 3134, a common high boron frit.
If you're willing to accept some dissolved borax in the glaze mix, it should be possible to use borax in a glaze recipe normally. However, there are some challenges.
Because borax is water soluble and because it tends to lose crystal water in storage, borax has the nasty property of setting like plaster when wet. Loose powder clumps together and turns rock hard.
Borax is also hard to get as a fine powder. Most glaze ingredients come in powders finer than 200 mesh. 20 Mule Borax, commonly available in North America as a laundry additive, comes in coarse particles approximately 30 to 80 mesh. If mixed as is in a glaze, the borax particles are easily visible and don't blend into the melt very well. (Though this may be an interesting effect for a specialty glaze.) These particles are glassy and somewhat amber - perhaps my source has some iron contamination?
I see no reason why this issue can't be solved by ball milling glazes containing borax. The ball mill would crush the clumps and grind down large particles. Sadly, I don't have access to a ball mill and cannot verify if this works.
Borax may be an interesting compliment or alternative to soda ash for carbon trapping. Carbon trapping is when soot is dissolved into the glaze during reduction firing. The resulting glaze is partially or completely black. Traditionally, soda ash is added to glazes to promote carbon trapping. Soda ash crystals grow on the surface of the glaze as the water evaporates. When fired, the ash melts early enough to absorb carbon from the kiln atmosphere.
Borax melts at 750C, 100 degrees colder than soda ash (850C), meaning it may trap carbon more aggressively. However, borax is not as soluble as soda ash, so not as much can be used. Perhaps a combination of borax and soda ash can carbon trap better than pure soda ash?
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