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Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.
Key phrases linking here: crystallization, devitrification, devitrify - Learn more
When ceramic melts are cooled they prefer to solidify as an organized molecular structure. Given sufficient time and sympathetic chemistry, they will form a crystalline structure. But if cooling is faster they solidify as a glass.
Crystals can grow in cooling glaze melts if one or more of the following conditions are present: the glaze melt is fluid, cooling rate is slow, oxides that like to form crystals are present (e.g. ZnO, TiO2), oxides that can form crystals are present in high proportions (e.g. CaO, Fe2O3), oxides that stiffen the melt are not present or present in low percentages (e.g. Al2O3, Zr, MgO). Crystals are normally silicate or borate compounds, thus SiO2 and B2O3 need to be present in significant amounts. Crystals can be seeded by incorporating them in a glaze batch. Glazes not normally prone to crystallization can sometimes be partially crystallized by slow cooling and glazes prone to crystallization can often be quickly cooled to prevent it.
Crystallization can be highly decorative (thus sought-after by potters) but is difficult to maintain consistency (and is thus often used in one-of-a-kind art ware). Potters go to great lengths to achieve reactive glazes and crystallization is a prime mechanism. We have seen potters use glaze recipes with as much as 13% tin oxide, an incredibly expensive material - the ecstasy of the visual aesthetics giving them amnesia of the cost!
Unwanted crystallization occurring in a glaze during cool-down in the firing is called devitrification, it spoils gloss surfaces and can be a real plague to industry. It can be dealt with by faster cooling or adjustments to chemistry (e.g. higher Al2O3, switching some CaO for MgO, reducing B2O3).
Very glossy or well-melted glazes can be subject to this because they likely either contain a lot of SiO2 (which combines with other oxides to form silicate crystals) or have a very fluid melt (which enables crystals greater freedom to form). When devitrification is desired it is simply called crystallization. The chemistry of the host glaze is the key factor since it determines the amount of melting and the presence of oxides that are crystal-friendly or impede crystallization (e.g. Al2O3, MgO). Many mattes are simply glossy glazes in which the entire surface has been invaded by micro-crystals. Purely decorative highly crystalline glazes are almost always high in Na2O and thus almost always craze badly.
A dramatic example of crystallization can be demonstrated by melting (and cooling) a powdered mix of 50:50 Ferro frit 3134 and cobalt oxide in a crucible at cone 6. The frit is a very active melter (it contains no alumina) and the cobalt is also an active melter, together they can work real magic!
Most artists and potters want some sort of visual variegation in their glazes. The cone 6 oxidation mug on the right demonstrates several types. Opacity variation with thickness: The outer blue varies (breaks) to brown on the edges of contours where the glaze layer is thinner. Phase changes: The rutile blue color swirls within because of phase changes within the glass (zones of differing chemistry). Crystallization: The inside glaze is normally a clear amber transparent, but because these were slow cooling in the firing, iron in the glass has crystallized on the surface. Clay color: The mugs are made from a brown clay, the iron within it is bleeding into the blue and amplifying color change on thin sections.
Two clear glazes fired in the same slow-cool kiln on the same body with the same thickness. Why is one suffering boron blue (1916Q) and the other is not? Chemistry and material sourcing. Boron blue crystals grow best when there is plenty of boron (and other power fluxes), alumina is low, adequate silica is available and cooling is slow enough to give them time to grow. In the glaze on the left B2O3 is higher, crystal-fighting Al2O3 and MgO levels are a lot lower, KNaO fluxing is significantly higher, it has more SiO2 and the cooling is slow. In addition, it is sourcing B2O3 from a frit making the boron even more available for crystal formation (the glaze on the right is G2931F, it sources its boron from Ulexite).
This glaze has a significant amount of cobalt carbonate and during cooling the excess is precipitating out into pink crystals during cooling in the kiln. This effect is unwanted because in this case since it produces an unpleasant surface and color (the photo does not clearly show how pink it is). This problem can be fixed by a combination of cooling the kiln faster, increasing the Al2O3 content in the glaze (it stiffens the melt and prevents crystal growth) or firing lower.
Both of these mugs were soaked 15 minutes at cone 6 (2200F), then cooled at 100F per hour to 2100F and soaked for 30 minutes and then cooled at 200F/hour to 1500F. This firing schedule was done to eliminate glaze defects like pinholes and blisters. Normally the GA6-A glaze crystallizes (devitrifies) heavily with this type of firing, but an addition of 1% tin oxide to the one on the left has prevented this behavior.
The 80:20 base GA6-A Alberta slip base becomes oatmeal when over saturated with rutile or titanium (left: 6% rutile, 3% titanium; right: 4% rutile, 2% titanium). That oatmeal effect is actually the excess titanium crystallizing out of solution into the melt as the kiln cools. Although the visual effects can be interesting, the micro-crystalline surface is unpleasant to touch and susceptible to cutlery marking and leaching (not as stable or durable as in glazes which are pure amorphous glass). For functional ware, rutile glazes are among the most troublesome to keep consistent, one way of avoiding problems is keeping the percentage as low as possible while still getting the desired variegation (of course that will vary depending on the melt fluidity of the glaze, more highly fluid ones can handle more rutile or titanium).
Crystallization (also called devritrification). You can see the tiny crystals on the surface of this copper stained cone 6 glaze (G3806C). The preferred orientation of metallic oxides is crystalline. When kilns cool quickly there is simply not enough time for oxides in an average glaze to organize themselves in the preferred way and therefore crystals do not grow. But if the glaze has a fluid melt and it cools slowly through the temperature at which the crystals like to form, they will. There is another issue here also: There are tiny dimples in the surface. This is because copper carbonate was used here instead of copper oxide. During firing, it generates carbon dioxide (because it is a carbonate) that bubbles out of the melt, leaving behind dimples that may or may not heal during cooling.
This glaze consists of micro fine silica, calcined EP kaolin, Ferro Frit 3249 MgO frit, and Ferro Frit 3134. It has been ball milled for 1, 3, and 6 hours with these same results. Notice the crystallization that is occurring. This is likely a product of the MgO in the Frit 3249. This high boron frit introduces it in a far more mobile and fluid state than would talc or dolomite and MgO is a matting agent (by virtue of the micro crystallization it can produce). The fluid melt and the fine silica further enhance the effect.
A melt fluidity comparison between two cone 6 matte glazes. G2934 is an MgO saturated boron fluxed glaze that melts to the right degree, forms a good glass, has a low thermal expansion, resists leaching and does not cutlery mark. G2000 is a much-trafficked cone 6 recipe, it is fluxed by zinc to produce a surface mesh of micro-crystals that not only mattes but also opacifies the glaze. But it forms a poor glass, runs too much, cutlery marks badly, stains easily, crazes and is likely not food safe! The G2934 recipe is google-searchable and a good demonstration of how the high-MgO matte mechanism (from talc) creates a silky surface at cone 6 oxidation the same as it does at cone 10 reduction (from dolomite). However it does need a tin or zircon addition to be white.
Each potter using Tenmoku has their own preferences about how the glaze should look. Ron clearly likes the iron crystals to develop well on the edges of contours. He has learned how to walk a delicate firing and recipe balance to achieve this effect. If the percentage of iron is too high, or the glaze is applied too thin, reduction is too heavy or the cooling too slow there will be too muchy crystallization. If the iron is too low, cooling is too fast or the glaze it too thick it will be a solid black. Additionally, this effect depends on a glaze having a fluid melt (the iron is a strong flux), if the glaze is too thick it will run downward during the firing.
Well, actually they are not exactly the same. This is 80% Alberta Slip and 20% frit. But the frit on the left is Ferro 3195 and on the right is 3134. By comparing the calculated chemistry for these two we can say that the likely reason for the difference is the Al2O3 content. Frit 3134 has almost none whereas 3195 has 12%. Al2O3 stiffens the glaze melt, that impedes crystal growth. And it stabilizes the melt against running during firing. Frit 3195 is thus much more "like a glaze" than is 3134, it is what Alberta Slip needs to melt as a transparent glass under normally cooling in the kiln.
These tests of a recipe called "Strontium Crystal Magic". The potter tried it on different bodies and firings. But instead of producing the magic crystals like the pictures, the surfaces fired totally matte. Reasoning "why would anyone put a recipe on line that does not work", she blamed one of the materials. Others fed that with rumours of claimed issues in its consistency. Admittedly, this glaze is meant for layering over others - but the source did not say that. This underscores misguided trust in trafficked recipes that most often lack sufficient documentation. Crystal glazes, by necessity, need to have a high melt fluidity. The crystals develop best with a specific cooling curve having a controlled fall at a narrow temperature range. Cool faster, they don't grow, slower and they matte the entire surface. Other factors, like clay body and glaze thickness are involved. People who post glaze recipes like this often do not document them well because they do not fully understand their mechanisms.
Iron is among the most powerful of fluxes in reduction firing. This is normally a glossy glaze, but the kiln was slow-cooled, resulting in total crystallization of the surface. The crystals are larger and layered at the neck. Their presence, as a thin layer on top, has completely matted the rest of the surface. Enough glaze ran downward off the piece that the vase was left sitting in a pool of molten glass.
GA6-A Alberta Slip base glaze (80 Alberta Slip:20 Frit 3134) fired with Plainsman slow cool cone 6 firing schedule on Plainsman M390 iron red clay. If this is cooled at normal speed, it fires to a glossy clear amber glass with no crystals.
Iron oxide is an amazing glaze addition in reduction. Here, I have added it to the G1947U transparent base. It produces green celadons at low percentages. Still transparent where thin, 5% is producing an amber glass (and the iron is showing its fluxing power). 7% brings opacity and tiny crystals are developing. By 9% color is black where thick, at 11% where thin or thick - this is “tenmoku territory”. 13% has moved it to an iron crystal (what some would call Tenmoku Gold), 17% is almost metallic. Past that, iron crystals are growing atop others. These samples were cooled naturally in a large reduction kiln, the crystallization mechanism would be much heavier if it were cooled more slowly.
These glazes are both 80% Alberta Slip, but the one on the right employs 20% Ferro Frit 3249 accelerate the melting (whereas the left one has 20% Frit 3134). Even though Frit 3249 is higher in boron and should melt better, its high MgO stiffens the glaze melt denying the mobility needed for the crystal growth.
This is an example of crystallization in a high MgO matte. MgO normally stiffens the glaze melt forming non-crystal mattes but at cone 10R many cool things happen with metal oxides, even at low percentages. Dolomite and talc are the key MgO sources.
The base glaze (inside and out) is GA6-D Alberta Slip glaze fired at cone 6 on a buff stoneware. However on the outside the dried glaze was over-sprayed with a very thin layer of titanium. The dramatic effect is a real testament to the variegating power of TiO2. An advantage of this technique is the source: Titanium dioxide. It is a more consistent source of TiO2 than the often-troublesome rutile.
This high boron cone 04 glaze is generating calcium-borate crystals during cool down (called boron-blue). This is a common problem and a reason to control the boron levels in transparent glazes; use just enough to melt it well. If more melt fluidity is needed, decrease the percentage of CaO. There is a positive: For opaque glazes, this effect can actually enable the use of less opacifier.
Closeup of a crystalline glaze. Crystals of this type can grow very large (centimeters) in size. These grow because the chemistry of the glaze and the firing have been tuned to encourage them. This involves melts that are highly fluid (lots of fluxes) with added metal oxides and a catalyst. The fluxes are dominated by K2O and Na2O (from frits) and the catalyst is zinc oxide (enough to contribute a lot of ZnO). Because Al2O3 stiffens glaze melts, preventing crystal growth, it can be almost zero in these glazes (clays and feldspars supply Al2O3, so these glazes have almost none). The firing cycles involve rapid descents, holds and slow cools (sometimes with rises between them). Each discontinuity in the cooling curve creates specific effects in the crystal growth. These kinds of glazes are within the reach of almost anyone today since electronic controller-equipped kilns are now commodity items and anyone can fiddle with the chemistry and manage the testing of glazes in their insight-live.com account.
Several things are needed for high silica glazes to crystallize as they cool. First a sufficiently fluid melt in which molecules can be mobile enough to assume their preferred connections. Second, cooling slowly enough to give them time to do this. Third, the slow cooling needs to occur at the temperature at which this best happens. Silica is highly crystallizable, melts of pure silica must be cooled very quickly to prevent crystallization. But Al2O3, and other oxides, disrupt the silicate hexagonal structure, making the glaze more resistant to crystallization.
Both mugs use the same cone 6 oxidation high-iron (9%), high-boron, fluid melt glaze. Iron silicate crystals have completely invaded the surface of the one on the right, turning the gloss surface into a yellowy matte. Why? Multiple factors. This glaze does not contain enough iron to guarantee crystallization on cooling. When cooled quickly it fires the ultragloss near-black on the left. As cooling is slowed at some point the iron will begin to precipitate as small scattered golden crystals (sometimes called Teadust or Sparkles). As cooling slows further the number and size of these increases. Their maximum saturation is achieved on the discovery, usually by accident, of the exact temperature they form at (normally hundreds of degrees below the firing cone). Potters seek this type of glaze but industry avoids it because of difficulties with consistency.
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
These two mugs have the Alberta Slip base cone 6 GA6-A glaze on the inside and GA6-C on the outside (it just adds rutile to GA6-A). The left one was cooled normally (kiln off at cone 6 after soak). For the mug on the right, the kiln was soaked for half an hour at 1800F on the way down to develop the rutile blue glaze on the outside. But during this period crystallization occurred on the inside also. This provides an insight into my this GA6-A base hosts floating blue effects but GA6-B does not: The amount of Al2O3 is much lower, that improves melt fluidity and acts as a catalyst for crystallization.
It is about the oxide chemistry, as shown calculated below the recipes in my account at insight-live.com. These glazes are fired at cone 6 using the C6DHSC schedule (we are focussing on the amber glossy glaze on the insides of the mugs). Most oxides want to form silicate crystals (combine with SiO2) as the glaze cools (if the cooling ramp is slow enough), iron oxide is not the least of these. Alumina (Al2O3) stabilizes the melt, that means it helps the melt to solidify as a glassy solid, not a crystalline one (thus, it does not devitrify). Notice the two Al2O3 values (black-on-red numbers): The glaze on the right has much less. That is because Ferro Frit 3134 contains almost no Al2O3 (notice in the blue panel, only 2%). The alumina in the glaze on the left, sourced more abundantly by Frit 3195, readily releases in the melt, ready to take on its job: Stiffen it and impede the formation of iron silicate crystals during cooling to create a better glass.
An iron crystal glaze on a buff stoneware at cone 10R
Glass vs. Crystalline
In ceramics, understanding the difference between what a glass and crystal are provides the basis for understanding the physical presence of glazes and clay bodies.
Understand your a glaze and learn how to adjust and improve it. Build others from that. We have bases for low, medium and high fire.
Boron blue is a glaze fault involving the crystallization of calcium, boron and silicate compounds. It can be solved using ceramic chemistry.
A type of ceramic glaze made by potters. Giant multicolored crystals grown on a super gloss low alumina glaze by controlling multiple holds and soaks during cooling
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to impart a practical level of strength and durability for the intended purpose.
|By Tony Hansen|
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