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

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...

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?

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.

WOW! Thanks you are an awesome fountain of chemical knowledge.

Barium Carbonate is added to clay and glazes in small amounts (0.1% to 0.8%) to remove soluble phosphates. So this can be an small addition to an Iron Phosphate glaze to prevent it from hardpanning.

http://digitalfire.com/4sight/hazards/ceramic_hazard_the_use_of_bar...

It creates barium sulfate whose Ksp is 1.08×10^-10, which potters call insoluble.

Compare this to table salt (NaCl) with a Ksp of 38.65 at 25 C room temperature, multiplied by an atomic weight of 58.44, that means almost 767 grams dissolve in a liter of water.

Salt is 386 billion times more soluble in water than barium sulfate.

So actually a small amount of barium dissolves in water, but a Ksp of 1.08×10^-10 is so small that it's safe for hospitals to give patients a lot of barium sulfate to drink as an x-ray contrast. Virtually no barium is absorbed, but barium is very dense, so it blocks the x-rays thus outlining the edges of your stomach and intestines.

If anyone wants more information on flocculating cations (Ca+, Mg+, Na+ and K+) this is an interesting University of Arizona PowerPoint presentation about flocculation of soil / mud.

You want a flocculated garden, so bentonite and calcium chloride are a gardener's friend.

Put into a slightly different context it can help make these processes in clay and glaze more understandable.

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&sour...

I'm not sure what advantage it would have over blend of bone ash and alumina. Phosphoric acid is expensive compared to these ingredients. As for replacing silica, The cost would be prohibitive, and chemically it would not seem to make sense.

So far replacing silica with alumina phosphate makes exactly the same glaze but often more translucent and with a change in the glaze color.

I'll post photos of the test tiles after I remake more glazes with this substitution.

These results are not surprising because two chemically different isoelectric molecules could be described as two boxes which have the same shape and bonding structure on the outside of the box, as far as other molecules are concerned. But since the inside of the box has a completely different structure it will reflect and refract light differently.

It seems like the original structure of the original molecule would break down in the melt and become part of a complex phosphate glass, just as silica forms complex silicates. So maybe you could get pretty much the same effect with bone ash and aluminum hydroxide in equivalent molar amounts and cut down the calcium in the other ingredients.. 

Have you replaced the silica in a 1:1 ratio in your glaze? What do you figure the cost difference is.

Most ceramic materials gave off a lot of energy as they formed. So you have to apply a lot of heat to break them apart. The energy needed to make a reaction happen is called Ka by chemists.

When iron spontaneously "rusts" into iron oxide it gives off energy with this equation moving to the right.

4 Fe + 3 O2 = 2 Fe2O3 + heat

If I want to make this equation move to the left so I get back to metallic iron and free oxygen, I have to add a tremendous amount of heat or energy to make this happen.

If I heat my kiln up to cone 10 (1300 C), I still don't have enough energy to break the Aluminum Phosphate box into parts. I need to get to the decomposition temperature of 1800 C. Alumina and Silica are similar. If I put alumina and silica in my kiln at cone 10, I still don't have enough heat to even link these two molecules aka "boxes" into aluminum silicate. I need to get the heat up to 1600 C.

Al2O3 + SiO2 = Al2SiO5

To break alumina and silica down into aluminum silicon and oxygen I would need to reach much, much higher temperatures still.

If I add some Calcium Carbonate to the Alumina and Silica I can get them to melt together at only 1170 C, around cone 5, which is within my kiln's capability.

http://digitalfire.com/4sight/material/calcium-aluminum-silicate_eu...

CaCO3 + Al2O3 + SiO2 = CO2 + Ca3Al2(SiO4)3

This reaction forms calcium aluminum silicate, also called Grossular. If I slow-cool grossular under tremendous pressure it will form a crystalline structure called a garnet.

This is just one of many eutectics we have to choose from in ceramics.

http://digitalfire.com/4sight/material/eutectics_2755.html

Now, back to aluminum phosphate.

The atomic weight of hydrated aluminum phosphate AlPO4-1.5H2O is 148.98 grams per mole. Silica SiO2 weighs only 60.08 grams per mole. So I would replace each gram of Silica with 2.48 grams of  Hydrated Alumina Phosphate.

When I replaced each gram of silica with one gram of hydrated alumina phosphate in Magruder's Red, the test tile looks just like it doesn't have enough silica. Using the 2.48 replacement ratio, the glaze looks like it should, but with different coloration.

Could aluminum phosphate form in my glaze with bone ash and alumina? To a very limited extent. The melting temperature of bone ash (calcium phosphate) is 1670 C which is again higher than cone 10. Our kilns don't reach high enough temperatures to fundamentally reform the ingredients we add. A better way to look at what comes out is an aggregate like cement, or the way egg holds a meat loaf together.

This is an electron micrograph of fired enamel, glaze, slip and clay. 

It's too small to see well, but you can see the unmelted aggregate materials held together by the melt.

If your glaze ingredients were ground more coarsely you wouldn't need an electronmicrograph to see how many of the glaze ingredients don't actually melt. You'd clearly see that your glaze, like your clay, bisques and at higher temperatures densifies.

The particles of aluminum phosphate I didn't grind well enough remain little white specs in the glaze.