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All About Pottery Glazes

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!!!

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?

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.

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



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



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.

Solubility

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

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

Handling soluble materials

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



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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Have you ever come up with a good idea while working with your handmade pottery or ceramics and thought that you would like to share it with others? You have? Well, why not send it to us and we will add it to the tips page for all to see.

Handmade pottery can be a very gratifying hobby that produces fun and satisfying results. For many people it's an enjoyable release that is created by working an inanimate mound of clay into a beautiful work of art that you made through your artistic abilities.

The best way of starting out is to take a few lessons. You will probably waste quite a bit in materials when you first get started. Figuring out how to truly make handmade pottery correctly and shape into what you want it to be can be quite an ordeal. The different tools that a normal shop will have can be fun to try. You will soon see which ones you like to use the most and then when you are ready you will know which ones to buy.

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.