The New World of Crystalline Glazes: Developing Beautiful Crystals in Reduction

creber-new-695

shots of a plate with glaze containing titanium and cobalt, strike fired to cone 018, by Bill Boyd, Galliano Island, British Columbia, Canada.


It used to be thought that crystalline glazes were only possible in an oxidation atmosphere. But as many more potters become seduced by these intriguing glazes, new and exciting discoveries – including developing crystals in reduction – are being made all the time.

In today’s post, crystalline potter Diane Creber explains the basics of growing crystals and how crystalline glaze potters have been recently experimenting using reduction to enhance the pre-formed crystals in their glazes. Plus she shares a couple of great crystalline glaze recipes and a crystalline firing program for a digital controller.   – Jennifer Harnetty, editor.


How to Develop Beautiful Crystals in Reduction

by Diane Creber

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 (see “Reduction Firing Crystalline Glazes” below).

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.

crystalfeat-300x145For 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.

 


If you enjoy crystalline glazes, check out Glazes and Glazing, in which William Sawhill,
Sumi von Dassow and William Schran all share their secrets for beautiful crystals.
Start exploring the world of crystalline glazes now!


Bottle with Gold Stuff Glaze (see recipe), fired in oxidation, by Diane Creber, Wilton, Ontario, Canada.

Bottle with Gold Stuff Glaze (see recipe), fired in oxidation, by Diane Creber, Wilton, Ontario, Canada.

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.

Bottle with Gold Stuff Glazem fired in oxidation, then refired to 1500 degrees F and reduced until the kiln cooled to 1250, by Diane Creber.

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” below).

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.

Covered Jar with Gold Stuff Glaze, fired in oxidation to grow crystals, then reduced from 2000-1850 degrees F, by John Tilton, Alachua, Florida.

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.

Screen Shot 2016-04-19 at 10.59.31 AMReduction Firing Crystalline Glazes

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.

Screen Shot 2016-04-19 at 10.59.18 AMAt 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 below), 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 color.

the author Diane Creber is the author of Crystalline Glazes. See www.wiltonpottery.ca for more information.

 


This post was excerpted from Ceramics Monthly magazine’s September 2010 issue. To get great content like this delivered right to your door, subscribe today!


Comments
  • Put your best vase forward

    You are invited to submit up to 3 pieces to be considered for an all crystalline glaze exhibition called Frozen In Fire – Crystalline Waves. In keeping with the NCECA theme TIDAL FORCES: The Next Wave; reduction or alternative fired crystalline glazed pots are encouraged but not required.

    The exhibition will be held at the Dunedin Fine Art Center in Dunedin Florida near the 2011 NCECA conference in Tampa. It will run from March 25 through May 1, 2011. The deadline to submit entries is December 15, 2010.

    Jurors, Diane Creber, Ginny Conrow, and John Tilton will be looking for excellence in glaze, form, and innovation to put our best face forward in an exhibition that will be enjoyed by the local community of Pinellas county as well as the most critical minds in the clay world from all over the country attending the 2011 NCECA conference in Tampa.

    Please email crystallineglaze@gmail.com for more details.

  • @Avi. Please enlighten those of us from Nigeria how we can attend such Exhibitions as this. I am a young teacher of Ceramics at Adeyemi College of Education,Ondo, Nigeria and I want to take my students to greater hieghts in Ceramics especially in Glazing as this has been the problem for a long time in my College before I joined in 2007. Any assistance would be humbly appreciated.

    Ganiyu S.O.
    sulayghan2000@gmail.com

  • I know I am like the neediest potter but if ANYONE HAS CONE SIX reduction or OXIDATION crystal glazes, i sure would love to see them here! Or on potterybasic or Clayarts? (or is it potterybasics, a group on Yahoo! COME ON DOWN THERE! And if you want to be in a reduction cone six group, POST HERE and I will invite you!

  • Roxanne – Check out this post for Cone 6 Crystal info:
    /ceramic-glaze-recipes/glaze-chemistry-ceramic-glaze-recipes-2/cone-6-crystalline-glazes-the-secrets-to-developing-crystals-at-mid-range/

  • Willard Spence has in his odd little book, “The Potter’s Oddyssey” a number of interesting recipes for cone 1 and cone 6 crystalline glazes. As I recall, most of them are aventurines (iron-saturated glazes) but they might be adaptable to zinc crystal formulas, yielding some interesting hybrids. Spence also details the making of his own frits in a home-built furnace.

    I used to get cone 10 zinc/titanium crystals in my old salt kiln, small but a pretty golden color.

  • Why do all crystalline glaze lovers make the same kind of pots, huge fat bottles with tiny necks ?
    Also, why do they always show HUGE crystals that look like skin disease ? Any glazes with small crystals ? What would you change in those glazes to form only small size crystals ?
    Sorry to sound so negative, the positive thinking squad will be over me in no time to bash me into silence! Don’t they know that interesting ideas come from discussion ?

  • Not sure if this link will work, but here’s a pic of my favorite new piece by Marsha Silverman who does amazing small matt crystals on lovely forms. If not, check out http://www.marshasilverman.com
    Not all crystalline glazers make the same kind of pots… check out this one for more beautiful forms & subtle crystals: http://www.conrowporcelain.com
    or see what I do at http://www.greendalepottery.com
    One thing about cone 6 crystals, many have started here, but not been happy with the depth of the results until moving up to cone 10 or 11. Just sayin.
    Holly

  • @Genevieve. If you look at the first pot (by John Tilton) in the article you will see many small crystals. The two pots in the article by Diane Creber have large crystals. Both pots have substantially the same glaze (Gold Stuff) but were fired using different firing schedules and different reduction methods. Slight changes in the ratio of frit to zinc to silica will produce very different crystal sizes.

    As to the shape of the pots. Crystalline glazes do look better on pots with smooth clean, simple lines. The glaze itself has a lot going on and gets lost on a pot with complicated shapes.

    Thanks for your input,

    Avi

  • Actually Genevieve, I agree. It does always seem to be those bottles. I think the 2nd example in particular ( the gold/tan piece) is not a good form. It is hard to mix great form and great glaze. That being said, I think the first example is one of the better crystalline glazes and form combinations I have seen. Small crystals, can also look like chicken pox and skin disease, by the way. Great post, great info ( especially the simple firing schedule) Thanks, Shawn

  • I have never tired Crystaline glazes but am geared to try it out in my new electric cone 6 kiln. Is there a firing schedule that has been proven out there? I loved the info provided; enough to move out of my comfort zone and give it a go! Being totally ignorant about this process, I also wonder if anyone has tried to catch run off with kiln furniture under the piece?

  • The bottle form is the style to display crystalline glazes at their best, simple forms to display beautiful crystals. Controling the firing cycle, and the soak duration will have an effect on the size, shape and colour saturation of the crystals. Large crystals which display growth rings can be created by several soaks at different temperatures, smaller crystals will have shorter soak periods. The secret to beautiful crystals is controlling and understanding the effect of fire, heat and saturation.

  • thanks for the post, very ineteresting, Does anybody know how to create crystals at low firing?
    thanks

  • You can make crystals happen at pretty much any temp, (see discussion posts with lots of info on low-fire crystalline on the Crystalline Glaze Forum at http://www.handspiral.com/ForumGate.htm ) but many crystal potters do come back to finding that the really scrumptious crystals with the greatest depth happen at cone 10-11…

  • hy im a ceramic teacher i would like to try cristalline glaces in a gas kiln whith a redaction firing is there a spacial technics thanks for help

  • There are several ways to fire a macro crystalline glaze in a reduction atmosphere – fire to top temperature in oxidation then a reduction atmosphere as you bring the temperature down or try a third firing cone 6 in a full reduction atmosphere – the colour of the final crystals will change as a result of the third firing. You can even raku crystalline glazes. It is important to have a reliable crystalline glaze before attempting experimenting with different firing atmospheres.

  • Hey there,
    I’m interested in doing some crystal firing but my question regards what kind of clay to use. I know most people say to use porcelain but I have some pieces thrown with stoneware that I would like to use and I was just wondering if you know how to change certain recipes for use with stoneware?

    Thanks,
    Kelly

  • Hey there, I’m interested in doing some crystal firing but my question regards what kind of clay to use. I know most people say to use porcelain but I have some pieces thrown with stoneware that I would like to use and I was just wondering if you know how to change certain recipes for use with stoneware? Thanks, Kelly

  • Yes i have a question about the glazes if anyone knows, do the bright contrasting colors under the crystals come from the glaze, or is that a slip used on the pot before bisque? thanks, cole

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