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Mysterious Crystals In The Glaze
by John Britt

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All About Pottery Glazes

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I had been experimenting with glazes for a while, trying to get new and different colors and designs, when I noticed small lumps in the glaze on the sides of the pot I used to mix my glaze concoctions, and I just figured that it was either bugs that got trapped in the glaze or chunks of sawdust that had fallen in my uncovered bucket. I keep my glazes in a unheated shed in the winter and they are subject to freezing and after looking carefully at these lumps, I found out that they weren't lumps, but small flat hexagonal flakes of material which kept getting bigger and bigger. They were actually small crystals which floated on or just under the surface of the thickened glaze!!!

Flambe John Britt at http://www.johnbrittpottery.com/gallery.htm

Deciding to find out just what the heck they were, I fired several in a small bowl in an electric kiln, while opening it periodically during the firing I noticed that they foamed intensely around 600°F, indicating that moisture was burning off. They melted into a pool of glass around 1600°F, while crystals placed on the side of a pot and gently heated with a propane torch melted into a liquid and dribbled down the side. I decided that these must be sodium borate crystals, which come from the glaze mixture as it cooled. You can get rid of them by pouring them in a sieve and dissolving them in warm water and then pouring the water back into the glaze, but I decided to glaze and fire a pot with as many crystals on it as possible. This produced a magnificent flambé glazed bowl, the crystals apparently fluxed out, melting down the side creating the effect. The mystery crystal is called a sodium borate crystal, producing a beautiful"Flambé" effect on my pottery bowl!! The picture above shows you the color and design that I found. Pretty huh?

Flambe Crystal 1

These 6’ x 3’ x 1/8 inch crystals above were found in Red Flambé slurry . I still didn't know for sure that the crystals were the primary cause of the variegated flambé effect rather than the way the piece was fired, so I fired two identical pieces on the same shelf, one glazed with a fresh batch and the other with the crystallized batch. The effect was only reproduced on the piece with the crystallized mixture, which told me that it was the cause of the variegated flambé surface.

The crystals mainly occurred in glazes with borates, or with borates and lithium carbonate, and varied from 1/2 inch thick by 2 inches in diameter to some that were so small it felt like sand in the glaze. Others covered the top of the 5 inch container with a thin body of crystals. Most were an opaque white but a number were red from iron oxide or green from copper carbonate being caught up in the crystal. To isolate their effect, some of the larger crystals were placed on the sides of pots and fired to Cone 10.

Flambe Crystal 2

This crystal was found in Jeff’s Red glaze measuring ½” x ½” x ¼”.

Flambe Crystal 3

The bowl with crystal’s found in Jeff’s Red placed on rim, reduction, porcelain, fired to cone 10.

John Britt's Flambe Bowl

The stem cup was coated with a fresh mix of Jeff’s Red, fired in reduction to cone 10, on porcelain.

I have found that the crystals not only occur mainly in batches of copper red glazes, including Jeff’s Red, Mary’s Surprise, Purple Red, Splotchy Lavender and Purple Sludge, but also in Matte Turquoise and Starshine, as well as many of my raku glazes. Each recipe contained borates, either from natural sources like gerstley borate or its substitutes or frits, demonstrating that many frits are slightly soluble in water.

It is evident that the flambé effect in Jeff’s Red was caused by soluble materials migrating into crystals as the slurry cooled. This meant that the mesh size of soluble color enhancing oxides, like lithium, sodium and boron, was increasing and that new combinations of materials were forming with higher concentrations of colorants, like copper and iron. Firing them on the pot created areas of greater concentration of those materials.


All ceramic materials have physical and chemical properties that will have an effect on glazes, including mesh size, melting and volatilization points, surface tension, solubility, etc. One key property is solubility, which is the ability of a material to go into solution at a given temperature. Soluble and slightly soluble compounds include borax, soda ash, pearl ash, epsom salts, wood ash, borate and its substitutes, copper sulphate, nepheline syenite, lithium carbonate and barium carbonate. Solubility usually varies with temperature. .81 oz of borax will dissolve in 3 1/2 oz of water at 140°F. But at 32°F only .04 oz will dissolve. So in a glaze made up at 140°F and slowly cooled toward freezing, .77 oz of borax will condense out. Also, if the volume of water is reduced, from evaporation, some of the solute will fall out of solution. But rather than fall to the bottom of the bucket, the charged ionic particles can be attracted to a seed crystal or almost any small clump of material and, as the temperature drops, more and more ions are attracted in a regular pattern, creating a crystal. This is, a dynamic process where some materials react and fall out of solution which allows more room for other materials to go into solution.

Flambe Crystal 5 Flambe Crystal 6 Flambe Crystal 9

Crystals found in Starshine glaze slurry, ½” x ½”x ½”.

Flambe Crystal 7

Bowl with dried mix of Starshine glaze, reduction, porcelain, fired to Cone 10.

Flambe Crystal 8

Bowl with fresh mix of Starshine glaze, reduction, porcelain, fired to Cone 10.

One interesting thing that I found was that crystals did not occur in all the glazes that contained borates and lithium oxide . Over half of the 80 copper red recipes tested had no crystals, suggesting that there are factors influencing crystal formation other than cooling, evaporation, and the availability of soluble borates, sodium and lithium oxide. One possibility could be variations in pH or the glazes.

The pH value of a solution refers to its acidity or alkalinity. This often stems from a material’s ability to go into solution rather than stay in suspension. Since multiple materials are used in a glaze, the pH of one material can affect the ability of other materials to go into solution. Barium carbonate is listed as insoluble in pure water but glaze slops can be slightly acidic and in those cases barium carbonate can be slightly soluble. Materials that are slightly soluble will dissolve in the glaze slop into extremely small charged particles.

These small dissolved particles can greatly affect the condition of the glaze slop, and may cause it to become flocculated or deflocculated and more difficult to handle. Plant or wood ash, which is high in soluble salts, if used straight from the hearth can easily deflocculate a glaze slop, making it too thin. To avoid this and provide consistency, the ash should be washed before being used. Flocculants, like epsom salts, cause the glaze slop to gel or thicken, which may make the application too thick, resulting in cracking as it dries. Flocculation and deflocculation are not bad in themselves, they are merely conditions of a glaze slop. Some glazes are purposely deflocculated to aid in spraying or brushing, or flocculated to help keep particles suspended. It only becomes a problem if it is out of control.

Flambe Crystal 13

Larger crystal found in slurry, 3” x 2 ½”x ½’.

Flambe Crystal 15

Soda ash crystals grown in saturated solution, 3” x 3” x 3”.

Flambe Crystal 14

Tri-sodium phosphate crystals grown in saturated solution, 3” x 1” x 1”.

Handling soluble materials

Frits, created partly to avoid solubility problems associated with some materials or to make toxic materials such as lead safer to handle, are manufactured by taking soluble materials and mixing them with clay, silica or a variety of other oxides, firing them to melting point and pouring them into water to make them shatter before grinding into a powder. Some frits, such as the alkali, remain slightly soluble, in spite of, being labeled as insoluble.

Soluble materials need careful attention. Those dissolved in the glaze will be lost if water is poured off to thicken the glaze. They will also migrate into the bisque ware as a glazed pot dries, which can affect the clay body because some of the fluxing oxides will remain there. They can also make the final look of the glaze difficult to control because the drying process often creates uneven concentrations of these materials, depending on where the water is evaporating. These are all sound reasons to avoid using them.

Flambe Crystal 11

Crystals found in Splotchy Lavender glaze slurry 2” x 1” x1”.

Flambe Crystal 12

Crystals found in Splotchy Lavender placed on rim, reduction, porcelain, fired to cone 10.

Crystals will also form if glaze slurries containing soluble materials are allowed to dry out completely. As the water evaporates the soluble ingredients form small crystals, which may be seen on the sides of the bucket and on the surface of the dried glaze. Once the glaze is fully dried you can add water and allow it to rehydrate. Using it without sieving or grinding up the chunks can result in some interesting surfaces. I have done this with carbon trap shinos and Starshine. Many potters may be experiencing this effect without realizing it, as we often allow our glazes to dry out when not in use. Then, in a rush to glaze something, we re-wet and apply them, often without sieving.

Glazing and firing is a complex process and discovering that crystals can grow in glaze adds yet another variable. We know that our glaze materials and kiln atmospheres are in constant flux, and the glaze slurry itself may be also. These crystal formations, some virtually invisible, could be causing migration of oxides within the slurries that dramatically affect the outcome of the glaze appearance. Since ancient pottery workers did not have modern heating technologies, it is possible that some unique historic glaze effects may have, accidentally been the result of crystallization of soluble oxides in their glazes.

With the knowledge of Flambé and being able to buy glazes that produce the Flambé effect, it is much easier to get the colors and design that we want. Once you glaze with these glazes you will be just like me, and can't hardly wait to open the kiln to see what you got this time!!! It is just like Christmas every time you open your kiln.

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When you get all set up, just enjoy the hobby and have fun at it. Some people get pretty serious and start selling their creations at craft fairs and small stores, but others just like to create items for themselves, relatives, and friends. Whichever kind of handmade pottery you desire to endeavor, enjoy the hobby and have fun doing it.

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