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Some ceramicists decorate pottery using soluble salts (usually chlorides and sulfates e.g. copper chloride, cobalt sulfate) which are used individually or mixed to produce different colors.

I have been exploring the use of these chemicals on porcelain. From the MSDS of each chemical I've become aware of the safety concerns and the proper precautions to take when mixing, applying, and firing them in a kiln.

But I don't know much about chemistry and do not understand the reactions when mixing these chemicals. Are there any chemicals which should absolutely not be mixed together? For instance, the Wikipedia article on Potassium permanganate says "Concentrated sulfuric acid reacts with $\ce{KMnO4}$ to give $\ce{Mn2O7}$, which can be explosive." Does this mean I should not mix Potassium permanganate with a sulfate like Copper Sulfate? In general, what happens when a sulfate is mixed with a chloride? Are there any general safety guidelines you could offer? Is there a fairly simple way to understand these interactions without the requirement of years of Chemistry study?

Apologies for my ignorance!

Some chemicals that ceramicists have used before:

Color

  1. gray
    • copper chloride (heavy application and heavy reduction can give pinks and reds)
    • palladium chloride
    • ruthenium chloride
    • selenium (selenous acid, selenium toner)
    • silver nitrate
    • tellurium chloride
    • vanadium (vanadyl sulfate, vanadium pentoxide)
  2. blue
    • cobalt chloride
    • molybdenum (molybdic acid)
  3. green
    • ammonium chromate
    • nickel chloride
    • potassium dichromate
    • sodium chromate
  4. brown
    • iron chloride (iron chloride emits heat when mixed with water so the water should be added gradually in small amounts)
  5. pink/purple/maroon
    • gold chloride (1-5% solution, adding either cobalt, manganese or
    • tellurium will give different shades)
  6. yellow
    • praseodymium chloride (very pale color)
  7. black
    • cobalt chloride (50% solution) and iron chloride (100% solution)
    • cobalt chloride (50% solution) and nickel chloride (50% solution)
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  • $\begingroup$ Are you more interested in the chemistry or the colourants in aqueous solution or in the ceramic glaze? $\endgroup$
    – Klaus-Dieter Warzecha
    Apr 24, 2014 at 7:23
  • $\begingroup$ Since ceramic glazes are usually handled and sprayed, I'm very hesitant to add any of these salts directly into the glaze or even clay body. I was thinking more of painting the salts and then firing, limiting exposure as much as possible. $\endgroup$
    – user1954085
    Apr 25, 2014 at 6:48

2 Answers 2

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Overall, always wear disposable gloves, goggles, and a dust mask. Add a lab coat, smock, or apron so as not to contaminate your clothing. Avoid skin contact, don't ingest and don't inhale (this includes the dust).

Manganese heptoxide is a special case with concentrated sulfuric acid. Avoid mixing strong oxidizers (permanganate, chromate/dichromate, perchlorate) with organics. NEVER screw around with chlorate salts. They are powerful oxidizers with very low kinetic barriers. Finely divided metals (especially titanium) can go to white-hot burning in open air. Titanium burns under air, carbon dioxide, nitrogen, water...

Palladium is a heavy metal, poison and tissue corrodant. A microgram of tellurium by any contact gives "tellurium breath"

Local contact with chromates can cause ulceration. Neodymium gives you purples, cerium yellow to orange. I seem to remember that cobalt blue (cobalt aluminate after firing) in tin oxide applied to white ware gives bubblegum purple.

Tin Glazing

Gold via the Purple of Cassius is very nice.

Here's a pretty! Suppose you mixed clay with hollow glass microbubbles, then fired. Would the clay foam to an interesting surface or whole body? This is the stuff, sold generally as lightweight resin filler, including hobby stores. Most prices are outrageous when sold by volume. Density of 3M Scotchlite K20 is 0.2 g/cm^3. Goggles and dust mask. Be careful or the stuff goes everywhere.

http://www.aircraftspruce.com/catalog/cmpages/bubbles.php
Good price.
http://solutions.3m.com/wps/portal/3M/en_US/Energy-Advanced/Materials/Product/?N=5002440+7570097&rt=r3

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  • $\begingroup$ Hi Al, This is a wonderful answer, thank you. I was not aware of the safety issues you mentioned- sometimes just looking at the MSDS of chemicals doesn't make it very clear which chemicals are dangerous vs. super dangerous. $\endgroup$
    – user1954085
    Apr 25, 2014 at 6:40
  • $\begingroup$ And microbubbles are a great idea. Lately ceramicists have been experimenting more with additives in clay bodies, from paper pulp to styrofoam beads. Maybe glass microbubbles would melt and fuse with the clay body at high temperatures, perhaps hardening the resulting ceramic and decreasing shrinkage. Might even influence ability to absorb thermal shock? $\endgroup$
    – user1954085
    Apr 25, 2014 at 6:46
  • $\begingroup$ Porcelain is glass-ceramic. Whole foamed body, added thin clay surface or shape decoration, or in the glaze are all interesting. K20 is E-glass borosilicate, 400 psi crush. There are some dozen compositions from fly ash to phenolics, with different size cuts and crush strengths/wall thicknesses. I use a stiff paste of K20 in architectural epoxy as two seal layers for messages in bottles. Sunlight eats organics, seawater eats metals, shingle beaches shatter bottles. A 100-year seal is challenging. $\endgroup$
    – Uncle Al
    Apr 25, 2014 at 22:29
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Sorry for resurrecting the topic, but original question wasn't fully answered.

What happens when a sulfate is mixed with a chloride? The answer for glazing is, nothing.

The exception would be mixing reducing and oxidizing salts, but there are no such sulfates or chlorides in your list. From you list, potassium dichromate, sodium chromate and exotic substances may be dangerous, as stated in selected answer.

Given basic protection from dust and swallowing, don't be afraid to experiment with commonly available salts. No one would sell you tellurium, while chlorates and permanganate are probably pointless in glazing.

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