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Comment by Norm Stuart on July 18, 2013 at 11:24am

You have to choose phosphate ingredients which are:

1.) stable at high temperatures, and

2.) very nearly insoluble, which chemists call having a very low Ksp in water.

You can easily find the Ksp of most chemicals using Google

http://www.google.com/search?q=%22aluminum%20phosphate%22%20ksp

Phosphates dissolved in water will combine with the flocculating Ca+ and Mg+ cations taking them out of solution as insoluble phosphates, thereby deflocculating the glaze and making it hardpan.

You can easily demonstrate this by stirring a little liquid phosphoric acid into a glaze to make it hardpan, even though this also releases flocculating H+ cations. It doesn't take much.

In contrast, adding some H+ cations in the form of muriatic acid (aka hydrochloric acid ak HCl) or vinegar (aka acetic acid aka CH3COOH) will flocculate a glaze. Chlorides, the Cl- forms soluble ionic compounds. This works until the H+ acid cations dissolve any glaze ingredients which release too many sodium or potassium cations.

Bone ash is the usual way potters add phosphorous as it meets both of these tests, a low ksp of 1.3 * 10^–26 at 25 C room temperature, and a melting temperature of 1670 C with cone 10 being only 1294 C.

Bone ash is Ca5(OH)(PO₄)3 with an atomic weight of 502.3 so multiplying this weight by the ksp at 25 C means only 6.5 x10-24 grams of bone ash will dissolve in a liter of water. That means only 0.00000000000000000000065 milligrams dissolve in a liter of water.

Aluminum phosphate is nearly the same, melting at an even higher temperature of 1800 C, with a ksp of 6.3 * 10-19. AlPO4-1.5H2O has an atomic weight of 148.97, so only 9.4 * 10^-17 grams dissolve in a liter. There will be more phosphate dissolved in the water than with bone ash, but it's still so small it's not relevant.

Iron Phosphate, which I'm also trying out, is not quite as favorable.  Iron Phosphate has a ksp of 6.3 * 10^^–19 and AlPO4-1.5H2O has an atomic weight of 150.8 grams so only 9.5 *10^-17 grams dissolve in a liter. In practice, replacing bone ash with this iron phosphate is slightly deflocculating so I needed to add back some extra Ca+ cations in the form of small sprinkle of calcium chloride pearls  which we keep around to flocculate glazes. I think this may also be explained by the possibility that the Iron Phosphate I bought may have other free phosphates, as it tastes slightly salty. (Don't worry, this Iron Phosphate is a food supplement for cattle and is also added to flour and rice to make it more nutritious)

http://www.amazon.com/Morton-7877-Safe-T-Power-Calcium-Chloride/dp/...

The potential downside of too much Iron Phosphate is it decomposes around 678 C, which is only cone 019. The free iron becomes red iron oxide, but two PO4 phosphates combine to form Phosphorous Pentoxide P2O5.

http://digitalfire.com/4sight/oxide/p2o5.html

The Phosphorous Pentoxide reacts with any metal oxides in the glaze like alumina, calcium, iron etc.

An excess amount of Phosphorous Pentoxide is not good as it will leave the glaze, less desirably finding the same ingredients to combine with in the kiln wall or elements or combine with moisture in the air to form phosphoric acid, which is just another reason we don't like breathing fumes from hot kilns.

There's a lot of useful information a potter can learn in this post if they haven't taken chemistry.  It may be too many new ideas to take in, all in one reading. But understanding these concepts will be rewarding in making glazes.

Comment by Christopher Cisper on July 17, 2013 at 11:51pm

I am totally amateur when it come to chemistry of this ilk. But will the phosphates gas off or just melt into the glass.  And if they gas off could that degrade the brick or the elements, could it aid in creating localized reduction? Just was curious. I am assuming that you grind the minerals before they go into the glaze, or maybe through a fritted glaze?

Comment by Norm Stuart on July 17, 2013 at 11:09pm

As I replace silica with aluminum phosphate in different glazes I'll find out if this is useful in a practical way to art ceramics. I suspect many PhD's in ceramic engineering could easily tell me a lot I don't know about how alumina phosphate compares with silica in a glaze/glass, apart from being more expensive. I suspect this is part of how the addition of phosphorous changes glaze color. 

I'm starting with a lithium fluxed glaze, Kate Magruder's Red, because I know lithium aluminum phosphate glass exists and can be doped with germanium or molybdenum to become  a superconductor, which for us would be like adding colorant oxides.

Today I replaced an equal weight amount of silica with aluminum phosphate in the recipe. But this is probably wrong as a mole of aluminum phosphate weighs 2.5x a mole of silica.

Aluminum phosphate by itself is temperature stable to 1800C, well beyond cone 10, so it will exist in the resulting glaze but will form additional bonds. But will there be a visible difference? I'll know more when the kiln cools.

There's also papers on aluminum phosphate glass because they're useful for lasers and optics.

http://www.lle.rochester.edu/media/publications/high_school_reports...

Comment by Robert Serva on July 17, 2013 at 6:07pm

Good valuable information, Thank you. We lean something new everyday.

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