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Comment by Robert Coyle on July 20, 2013 at 4:12pm

Thanks for the info Norm. I look forward to seeing the results of your tests.

Comment by Norm Stuart on July 20, 2013 at 10:35am

The electronmicrograph of clay, slip, glaze, and enamel was made by the US Department of Transportation.

Your method using aluminum, hydrochloric acid and TSP results in a mixture of aluminum phosphate and table salt, which is probably not what you want unless you're doing a salt firing, so you'll need to do some additional chemistry to take out the salt.

6 HCl + 2 Al = 2 AlCl3 + 3 H2

Na3PO4 + AlCl3 = AlPO4 + NaCl

Aluminum phosphate is not soluble in water, but hydrated aluminum phosphate does a really good imitation of being in solution. You could dry the result and try washing the salt out of the mix before the aluminum phosphate rehydrates.

You can purchase a fifty pound bag of sodium aluminum phosphate NaAlPO4 for about $45. It's called E541 or SALP and is used widely in the food industry as a baking powder and to emulsify cheese. But you'd have to break this chemical down somehow to AlPO4, as NaAlPO4 is not isoelectric with Silica, only AlPO4.

Comment by Robert Coyle on July 20, 2013 at 9:10am

That is an interesting micrograph. Was this a picture of the glaze you made with the aluminum phosphate?

Could you make this using aluminum cans dissolved in muriatic acid added to TSP solution, and then filter out the aluminum phosphate? The acid and TSP you could get at  a hardware store.

Comment by Norm Stuart on July 19, 2013 at 7:44pm

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.

Comment by Robert Coyle on July 19, 2013 at 9:52am

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.

Comment by Norm Stuart on July 18, 2013 at 10:15pm

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.

Comment by Robert Coyle on July 18, 2013 at 6:13pm

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.

Comment by Norm Stuart on July 18, 2013 at 1:07pm

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...

Comment by Norm Stuart on July 18, 2013 at 12:39pm

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.

Comment by Christopher Cisper on July 18, 2013 at 11:49am

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

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