|Monthly Tech-Tip |
Warping happens during the firing of ceramic ware when there is a high degree of vitrification or a shape is unstable. But warping is expected in translucent ware, it is just a factor that must be compensated for.
This normally refers to a body firing problem where vessels warp out-of-shape during firing. Warping often exhibits as out-of-round vessel rims or as sagging on overhung shapes (slumping). Any clay that is being fired to a vitreous condition is subject to warping (especially for pieces having unstable profiles). Clays that are being over fired can warp, often because a kiln controller takes the body higher than assumed. Clay bodies soften as they approach their melting point during firing. Porcelains are the most susceptible since they, by design, contain more feldspar. Thus for vitreous bodies, a balancing act must be done, firing as high as possible to get the most dense and strongest possible body while employing ware shapes and wall thicknesses that are resistant to warping. When using a body or process subject to this problem, it is important to have kilns that can fire evenly and consistently and use a body having repeatable firing properties.
Even earthenware, although not fired anywhere near vitrification, can warp if ware is extremely overhung or ware is very thin. In industry, highly vitreous ware (e.g. bone china) is fired in setters that hold its shape during firing (the glaze firing is done later at a much lower temperature).
Some google searches will turn up articles listing a variety of factors that determine susceptibility to warping. But such lists can be misleading. All other factors combined are not nearly as important as one: The percentage of feldspar. It is the melter, its percentage is a balance between the degree of maturity desired and the resistance to warping needed during firing. Different feldspars have different melting powers, but as different as you might assume. It is also worth mentioning that some kaolins contain feldspar (e.g. British kaolins). These can require considerably less feldspar yet produce a body having the same degree of vitrification (e.g. only 25% feldspar may be needed with Grolleg kaolin compared to 35% for New Zealand or an American kaolin). You may see some recipes with up to 50% feldspar, but these are impractical. Why? If 25% silica is needed, that leaves only 25% clay. No body with 25% clay will be plastic enough for even casting. These recipes might feature some added white bentonite, but even with 4% (which would still be incredibly expensive), they will have terrible plasticity. 35% feldspar (with an extra "kick" from 1-2% talc) will work just as well as 50%, that permits 40% kaolin. If glazes fit with 20% silica, then you have 45% kaolin. If you can drop thermal expansion of glazes and get away with 15% silica, you have 50% kaolin in the recipe. Adding bentonite to a 50:15:35 kaolin:silica:feldspar body can produce a super plastic body that you can throw two-foot-high vases from! Higher kaolin bodies convert more needle-shaped mullite crystals during firing, giving extra stability against warping.
Many stoneware and earthenware clays, that are mined and processed for industry and hobby, contain enough natural feldspar that they can be blended into bodies requiring zero feldspar mineral in the recipe. In fact, some natural clays actually over-fire at typical stoneware temperatures (e.g. cone 4-8), so they must be combined with more refractory materials (e.g. kaolin, silica) to produce a body that does not warp too much.
Translucency and warping go together. Translucent bodies must be highly vitreous, that means they will warp. Feldspar is the melter that make that happen. More melting makes more translucence. In industry it is common to take measures to support ware during firing (much more extreme than what a potter would do). Refractory bone china plate setters, for example, can take more manufacturing expertise that the plates themselves. Of course, where possible, ware shapes are designed to resist fired warping. So resistance-to-warping is a matter of balancing feldspar content, ware shape, cross section and method of kiln setting.
Again, be careful with articles you find online about porcelain formulation. They can be riddled with inaccuracies, exaggerations and blanket statements (from sources that should know better!). Porcelains do not warp more (or shrink more) on firing because they are white, they do because, in addition to employing white kaolins and bentonites, they have higher feldspar content. The base porcelain recipe is not 50% feldspar, 50% kaolin, 50% silica. It is 25 feldspar, 25 silica, 25 kaolin, 25 ball clay. Porcelains do not slump easier in the firing because they are less plastic, them slump because they have more feldspar (that higher feldspar makes less room for clay so they could be less plastic for that reason). More silica in a clay body “will” improve the glaze fit, not "may" improve the glaze fit (20-25% is a necessity). Fiddling with silica particle size might affect warping or slumping, but that is only to fine-tune, the feldspar content is the key (typically one trades feldspar for clay, leaving the silica at 20-25%). It is true that various silica grades are available at ceramic suppliers, but the particle size distributions of these do not vary nearly enough to make that an important factor. Again, the way to control slump is with the type and amount of feldspar. And, there are no "low-firing feldspars", the only way to do that would be to incorporate a frit for part of the feldspar. As noted, they can have somewhat different fluxing power (e.g. nepheline and soda feldspar vitrify better than potash), but the differences are not great. It takes 35-40% feldspar to vitrify an american kaolin at cone 5-6, it is not practical to go much lower since there is not enough room in the recipe for the needed clay and silica. A myth is that nepheline contains solubles that flocculate plastic bodies and make them unworkable, we have made thousands of tons of nepheline porcelains over 40 years, many kinds, it is the best feldspar.
Here is an example of how a profile having no inherent strength can warp during firing (the one on the left is just bisque fired, the one on the right is fired beyond zero porosity to achieve translucency). Two key factors contribute to this failure: This porcelain is highly vitreous. This shape is vulnerable to warping. If the lip were flared out, for example, it would have much more strength to stay round. If the porcelain was less vitreous it would warp less. Of the two factors, which contributes more to the warping for this specific piece? The shape.
These bars were fired at cone 10, they were straight when dry. The back one is a cone 10 Grolleg body, the front one is a cone 6 Grolleg body. This simple test is valuable to determine susceptibility to warping in porcelains. If the pyro-plastic deformation is too much, for example, the weight of a handle will pull the round rim of a mug into an oval shape, for example.
A porcelain mug has pulled slightly oval because of the weight of the handle. This happens in highly vitrified porcelains (e.g. translucent ones). The amount of feldspar or frit in the body determines the degree of maturity, the correct percentage is a balance between enough to get the maximum translucency and hardness but not so much that ware is deforming excessively during firing. This is Plainsman Polar Ice at cone 6, this degree of warp is acceptable and can be compensated for.
The goblet on the left is bending, not just because the clay is somewhat unstable at the temperature being fired, but because this shape is also inherently unstable. Where extreme shapes are prone to warping, ware must be made from clays that do not vitrify (that introduces issues of strength and functionality). In this case, the clay recipe is based on a terra cotta material that matures at a very low temperature. The problem was dealt with by employing a recipe of 60:40 clay:200# kyanite.
This is a low fire talc body. Many such bodies can survive to cone 6 and even higher, but not this one. The weight of the handle has pulled the lip completely oval.
When porcelains mature in the kiln they progress toward vitrification, getting softer. This simple test enables anyone to quantify the degree to which a porcelain is likely to warp. Bars of plastic clay almost never dry straight, so the measurement (in mm) to which they deviate from straight is recorded and the bar is mounted with the hump upwards. After firing the mm of firing deviation-from-straight are added to the dry value to derive a total pyro-plastic deformation measurement. This can be recorded as an absolute value for comparison with other clays or temperatures.
An example of a cone 10 porcelain that is over mature. It contains too much feldspar and is vitrifying so much that it is beginning to melt. The weight of the handle is pulling the lip into a oval shape, even though the hourglass shape of the piece should offer stability.
It may not melt, but will certainly warp and blister/bloat. If there is inadequate kiln wash it will stick to the kiln shelf.
When clay materials and bodies bubble as they melt or over fire. This normally happens in raw materials that contain particulates that produce gases during firing.
A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak.
Firing: What Happens to Ceramic Ware in a Firing Kiln
Understanding more about changes are taking place in the ware at each stage of a firing and you can tune the curve and atmosphere to produce better ware