Crystalline glazes are seductive and instantly capture the eye. How exhilarating to create a glaze that suggests floating galaxies, frosty windows, rare gemstones, or flowers. Enthusiasm for this glaze technique has greatly increased over the last few years, perhaps because of new advances in kiln technology and computer temperature controllers. Many more potters are now exploring the potential of this glaze technique, making new and exciting discoveries, including reduction firing (discussed on page 52).
However, before exploring the possibilities of crystalline glazes, the potter needs to understand the basics. The process of creating a crystalline glaze is more involved than working with a regular glaze as it has a tendency to run off the pot and the firing cycle is very dependent on accurate temperature measurements.
For the purpose of growing crystals, the kiln is taken up in temperature to around 300°F beyond the melting temperature of the glaze, (typically cone 6 to 12), which allows some of the zinc-silicate nuclei, or seed crystals, to dissolve. The crystals will grow from the few remaining nuclei.
After reaching top temperature, the kiln is cooled either by shutting it off or using a predetermined cooling rate, until it reaches the temperature where the crystals will grow. This is usually somewhere around 2100°F–1900°F. When the glaze is molten, all these ingredients float around in a liquid matrix. By holding the temperature at the point where the glaze is still slightly molten but just beginning to stiffen, the crystals form, with their size being determined by the amount of time they remain in this state. (Typically 3–5 hours). The kiln is then shut off and cooled naturally.
Making a Crystalline Glaze
There are three main ingredients in a crystalline glaze. Zinc oxide and silica make up approximately 25 parts each of the glaze and these are the crystal formers. They come together to form zinc silicate. The remaining 50 parts are the various ingredients that form or flux the glass melt, which can include boron, magnesium, calcium, lithium, sodium, and potassium. However, some of these ingredients are soluble in a glaze. Therefore, it is to the potter’s advantage to use a frit. When using a frit, a crystalline glaze could consist of only three ingredients: frit, zinc oxide, and silica. This combination, in the right proportions, will produce a crystalline glaze.
The most commonly used colorants are cobalt, copper, iron, manganese, nickel, and rutile. Less common are gold, silver, uranium, and some rare earth metals. These can be used alone or in combination with each other.
Let’s look at these ingredients individually. Most crystalline glaze recipes will call for calcined zinc oxide. Almost all of the zinc oxide you buy today is calcined, so there is no need to calcine it yourself. Check with your glaze supplier to ensure your zinc is calcined. If not, calcining is not a difficult procedure; place the zinc in a bisque bowl and heat it to cone 06–04, which means essentially putting it through a bisque firing. This will drive off any absorbed moisture. Zinc oxide absorbs moisture from the air so it must be stored in an airtight container. Calcined zinc oxide weighs less than uncalcined so make sure you are using calcined as recipes are based on the calcined weight. The silica should be 400 mesh (used for most glazes—but double check), because this particle size makes it easier to get a complete melt and no nucleation (the beginning of crystal growth, based around an unmelted zinc-silicate crystal).
There are many frit manufacturers on the market, which becomes confusing as frits are primarily made for industrial applications, and manufacturer names change. The most common frit used by crystalline potters is readily available and is called Fusion-75. It was formerly called Ferro 3110. If you ask your supplier for Ferro 3110 they usually know what you are asking for but sell you Fusion-75 (the same thing). I suggest using one frit and getting to know it well before trying another. Although different frits can give slightly different effects on the glaze, the greater variety comes from the minerals used. If using another frit, the glaze must be altered accordingly. You cannot substitute one frit for another without making adjustments based on the chemical composition of the frit.
Very small amounts of alumina or bentonite are often added to a glaze to help keep the materials in suspension (1–2%). These ingredients must be in very small quantities or the glaze will opacify and become a matte glaze. Adding a small amount (1%) of epsom salts to the glaze keeps the glaze flocculated.
Experimentation should be done first with the colorant. These are easier to adjust than other ingredients and can give the most dramatic results. Start with one colorant and add it to the base glaze in gradual increments. Also try mixing two or more colorants together. Final glaze slurries should be passed through an 80-mesh sieve.
The glaze should be thickly applied by pouring, dipping, or spraying, with a thicker application at the top of the pot, thinning towards the bottom. Once the pot is glazed, it is placed in the collection dish and put in the kiln ready for firing (see “Planning for Glaze Overflow” on page 51).
Striking is a glassblowing term that merely means re-heating a piece. Crystalline glaze potters have been using the term to apply to a firing procedure that also involves re-heating the piece in order to achieve different color effects. How great the change depends on the colorant. Some colorants react dramatically and others hardly change at all.
Once the pots are selected to be re-fired, the kiln temperature is taken up to cone 018 in a neutral or oxidizing atmosphere and shut off.
After the initial glaze firing, a cobalt glaze with 3.5% titanium dioxide results in dark blue crystals and a lighter blue to green background (above left). After the cone 018 refiring, the glaze has changed to a yellow/orange background with dark blue crystals and might also have an interesting smoky effect where the glaze is thicker (below left). Glazes with at least 3.5% titanium dioxide, along with any of the usual colorants like iron, copper, and cobalt, will respond well to a cone 018 refiring. The background gets darker and the crystals show more contrast with deeper colors.
Copper is very sensitive, so a glaze with as little as 3% copper carbonate and very little titanium changes dramatically. The first glaze firing results in silver/green crystals on a green background. Refiring to cone 018 results in a deep silvery dark green color with a mirror finish.
A glaze containing 5% titanium and 2.5% iron oxide appears an overall yellow and is rather bland looking after the initial crystalline firing. When re-fired to 018, it results in more contrast and a darker background. A yellow halo is produced around areas where the glaze pools, which isn’t there after the first firing.
These glazes are translucent, and the color of the clay will show through and affect the color of the glaze. Porcelain is the preferred clay, but white stoneware can be used as long as it is free of grog or large-grained silica, which could become nucleation sites for crystals. This would result in far more crystals than usually desired. When there is a large number of crystals, they butt up against each other and become a mass of crystals. This has the potential to make the glaze matte. (For more information on microcrystalline glazes, see Techno File on page 12.)
Porcelain that is slightly immature at cone 10 is preferred. Translucent clay is not ideal for crystalline glazes, though some use it. Most forms do not allow for translucency and making a super thin pot might cause it to crack because of the thick application of the glaze. These glazes are corrosive and can absorb alumina from the clay body if the clay starts to flux, which is what happens in a translucent body. Alumina affects crystal growth, and also can make the glazes go matte.
Not all shapes work well with crystalline glaze, either aesthetically or practically. Some consider these glazes to be rather glitzy so a pot with a simple but elegant form is usually most suitable. Throwing lines, appendages, handles, attachments or other adornments distract. Because the glaze runs, a cylindrical or bottle shape works well and also best displays the crystals. The glaze can collect on the inside of bowls and containers often creating unattractive matte areas that can be rough to the touch. However, with experimentation, the correct glaze thickness can be achieved in the bottom of plates and bowls and give pleasing results.
Interiors of vases or jars are sometimes glazed with a liner glaze, with the crystalline glaze applied only to the outside. When glazing the interior of a bowl, the glaze should be applied thickly to the rim area, becoming thinner on the sides and thinnest on the bottom.
Planning for Glaze Overflow
Precautions must be taken to protect kiln shelves from running glaze. The most typical method is to make either a one- or two-piece pedestal with a dish under it so that the top of the pedestal is the same diameter as the foot ring of the pot. The pot is glued to the pedestal and the excess glaze runs down the pedestal and into the collection dish. The glue may be a mixture of 50% kaolin and 50% alumina or it could be just kaolin mixed with a little water and Bondfast glue. The kaolin and alumina help separate the pot from the pedestal after the firing so the glue mixture should be applied thickly. I glue my pots to their pedestals before they are glazed.
Firing Crystalline Glazes
Most crystalline glaze firings take place in an electric kiln. It is possible to fire in a fuel kiln and that will be discussed later. The kiln must be capable of going to cone 10 or 11 without difficulty. A temperature controller is an asset. Kilns may be purchased with factory-installed controllers or a controller can be purchased separately and installed by an electrician. The controller must interface between the power source and the kiln’s main switching panel.
There are two types of temperature probes on the market: the S type and the K type. The S is more accurate, particularly for cone 9 or above, and is preferred by crystalline potters, although it is more expensive than the K.
Most new kilns come with three temperature probes so a potter can see what is happening in three different locations in the kiln. A laptop computer can be plugged into a jack provided by the manufacturer on some controllers. It can be attached directly to the processor and connected to a computer with KISS. (Kiln Interfacing Software System). This allows one to see graphically what is and has happened in three different areas of the kiln at the same time, and to make any necessary adjustments. During the firing, the actual time and temperature profile can be displayed, logged on the printer and saved on a disc, giving a complete picture of the entire firing sequence. Several kilns can be fired at the same time, and using a remote, the potter can monitor, graph and program firings while away from location. Another advantage is that firings can be repeated exactly, and the programs stored in the computer.
It is possible to fire without a temperature programmer by carefully watching the pyrometer and monitoring the kiln visually, but it requires the potter being present from the time the kiln reaches peak temperature throughout the crystal growing phase.
When the kiln has cooled to the point that the pots can be handled, they can be removed from the kiln. Lifting them by their collection dishes avoids the possibility of the pot separating from the dish and the dish falling back into the kiln.
Take a small hammer and tap around the collection dish to separate the pot from the dish. Usually the pot will separate. If not, use a glass cutter and score around the seam where the foot rim of the pot meets the top of the pedestal. Then take a small chisel and gently tap around this seam with a hammer.
Another method for removal is to use a propane torch, either the type used for spot welding or a crème brulé torch. The flame is aimed at the join line as the pot turns on a banding wheel. The pot quickly breaks away from its base. In both methods, it is necessary to wear safety goggles to protect the eyes from flying glass particles.
The bottom of the pot may have sharp edges of glass that can cut the hands. A silicon carbide grinding wheel can be used to remove the pieces of glass and smooth the bottom, and that may be all that is needed. To get a really smooth bottom, a diamond grinding pad is helpful. Whatever grinding method is used, wear a dust mask to protect your lungs from the dust.
Safety is always a factor when firing all kilns, but especially when fuel is introduced. These are flammable materials and one must never leave a kiln unattended throughout the reduction period. Good ventilation is a necessity and reduction should not be attempted unless one has a kiln hood to extract fumes, or the firing is conducted outdoors.
Crystalline glazes contain a large proportion of zinc oxide compared to standard glazes, sometimes comprising as much as 30% of the glaze. Zinc oxide is easily reduced to the metal at temperatures above 1742°F and the metal alone melts at 786°F and boils at 1697°F. Even slight reduction is sufficient to extract the oxygen which results in the loss of the zinc. Therefore any reduction at these temperatures or above will take away the oxygen, leaving a boiling metal. The boiling metal volatilizes and is lost. It is widely advised that zinc oxide be used only in fully oxidized or neutral firings. Because of this, it was long believed that crystals could not be grown in a reduction atmosphere.
Before electric kilns were available, crystals were grown in a neutral-to-oxidizing atmosphere in gas kilns. The crystalline glazed pots would be placed in saggars to protect them from any reduction that might occur. However crystalline glaze potters have been recently experimenting using reduction to enhance the pre-formed crystals, or introducing reduction during the final stages of the crystal growing phase, to completely alter the glaze.
The preference has been for electric kilns for firing crystalline glazes. Once potters realized that reduction can alter the glaze and create many more exciting effects, they started looking for ways to introduce reduction. With electricity, the ability to reduce is somewhat limited, although possible. But firing crystals in a gas kiln and using reduction as part of the process is also possible, using specific firing schedules.
At a recent workshop (Hamling studio, Warwick, New York), reducing in standard and custom-made electric kilns was demonstrated at two different temperatures (1500°F down to 1250°F and 2000°F down to 1850°F), and there were several firings with variations of these methods.
When reduction takes place after the crystals are grown and at temperatures below 1900°F, the zinc oxide is unaffected. This is sometimes done at the end of the initial glaze firing.
Crystalline glazes containing minerals that are affected by reduction (rutile, copper and iron) can be fired in oxidation and then refired in reduction. They no longer need their protective dishes and pedestals, which may be removed before firing. Before loading the kiln, a small fired dish is placed under the bottom peep hole inside the kiln. The kiln is fired to 1500°F and then is either shut off and reduction started as the kiln cools naturally down to 1250°F, or reduction can be introduced while the temperature drops to 1250°F using a temperature controller.
Reduction requires a fuel, and there are various ways the fuel can be introduced. One method of introducing fuel is to use an IV bag filled with the fuel (oil or denatured alcohol), available from some drug stores or a hospital supply store. When the kiln has reached the temperature to start reduction (1500°F), the bag is suspended above the kiln and the end of the tube is inserted into a protective ceramic sheath that goes into the kiln above the ceramic dish. The point of entry into the peep hole should be sealed with clay or fiber blanket to make the area airtight and to keep the sheath in place. Just below the bag on the tube is a small dial that regulates the rate of flow from the bag.
The difference between reducing at the higher and lower temperatures are obvious in the finished results. For instance, Gold Stuff Glaze (see recipe on page 52 and images on page 49), when fired in oxidation produces cream-colored crystals on a white background. When reduced at the higher temperature, the results are a blue background with gold colored crystals ringed in white. The same glaze reduced at the lower temperatures produces a deep purple background with olive green to gold crystals ringed in gold.
It is the firing that gives the diversity of glaze results, and using only one or two glazes gives the potter a whole palette of amazing colors.
the author Diane Creber is the author of Crystalline Glazes. See www.wiltonpottery.ca for more information.