Monthly Tech-Tip from Tony Hansen SignUp

No tracking! No ads! That's why this page loads quickly!

200 mesh | 325 mesh | 3D Design | 3D Printer | 3D Slicer | 3D-Printed Clay | 3D-Printing | Abrasion Ceramics | Acidic Oxides | Agglomeration | Alkali | Alkaline Earths | Amorphous | Apparent porosity | Artware | Ball milling | Bamboo Glaze | Base Glaze | Base-Coat Dipping Glaze | Basic Oxides | Batch Recipe | Bisque | Bit Image | Black Coring | Bleeding colors | Blender Mixing | Blisters | Bloating | Blunging | Bone China | Borate | Boron Blue | Boron Frit | Borosilicate | Breaking Glaze | Brick Making | Brushing Glaze | Calcination | Calculated Thermal Expansion | Candling | Carbon Burnout | Carbon trap glazes | CAS Numbers | Casting-Jiggering | Celadon Glaze | Ceramic | Ceramic Binder | Ceramic Decals | Ceramic Glaze | Ceramic Glaze Defects | Ceramic Ink | Ceramic Material | Ceramic Oxide | Ceramic Slip | Ceramic Stain | Ceramic Tile | Ceramics | Characterization | Chemical Analysis | Chromaticity | Clay | Clay body | Clay Body Porosity | Clay for Ovens and Heaters | Clay Stiffness | Co-efficient of Thermal Expansion | Code Numbering | Coil pottery | Colloid | Colorant | Cone 1 | Cone 5 | Cone 6 | Cone plaque | Copper Red | Cordierite Ceramics | Crackle glaze | Crawling | Crazing | Cristobalite | Cristobalite Inversion | Crucible | Crystalline glazes | Crystallization | Cuerda Seca | Cutlery Marking | Decomposition | Deflocculation | Deoxylidration | Differential thermal analysis | Digitalfire Foresight | Digitalfire Insight | Digitalfire Reference Library | Dimpled glaze | Dip Glazing | Dipping Glaze | Dishwasher Safe | Dolomite Matte | Drop-and-Soak Firing | Drying Crack | Drying Performance | Drying Shrinkage | Dunting | Dust Pressing | Earthenware | Efflorescence | Encapsulated Stain | Engobe | Eutectic | Fast Fire Glazes | Fat Glaze | Feldspar Glazes | Fining Agent | Firebrick | Fireclay | Fired Strength | Firing Schedule | Firing Shrinkage | Flameware | Flashing | Flocculation | Fluid Melt Glazes | Flux | Food Safe | Foot Ring | Forming Method | Formula Ratios | Formula Weight | Frit | Fritware | Functional | GHS Safety Data Sheets | Glass vs. Crystalline | Glass-Ceramic Glazes | Glaze Bubbles | Glaze Chemistry | Glaze Compression | Glaze Durability | Glaze fit | Glaze Gelling | Glaze laydown | Glaze Layering | Glaze Mixing | Glaze Recipes | Glaze Shrinkage | Glaze thickness | Globally Harmonized Data Sheets | Glossy Glaze | Green Strength | Grog | Gunmetal glaze | Handles | High Temperature Glaze | Hot Pressing | Incised decoration | Industrial clay body | Ink Jet Printing | Inside-only Glazing | Insight-Live | Interface | Iron Red Glaze | Jasper Ware | Jiggering | Kaki | Kiln Controller | Kiln Firing | Kiln fumes | Kiln venting system | Kiln Wash | Kovar Metal | Laminations | Leaching | Lead in Ceramic Glazes | Leather hard | Lime Popping | Limit Formula | Limit Recipe | Liner Glaze | Liner glazing | Liquid Bright Colors | LOI | Low Temperature Glaze | Majolica | Marbling | Material Substitution | Matte Glaze | Maturity | Maximum Density | MDT | Mechanism | Medium Temperature Glaze | Melt Fluidity | Melting Temperature | Metal Oxides | Metallic Glazes | Micro Organisms | Microwave Safe | Mineral phase | Mineralogy | Mocha glazes | Mohs Hardness | Mole% | Monocottura | Mosaic Tile | Mottled | Mullite Crystals | Native Clay | Non Oxide Ceramics | Oil-spot glaze | Once fire glazing | Opacifier | Opacity | Ovenware | Overglaze | Oxidation Firing | Oxide Formula | Oxide Interaction | Oxide System | Particle orientation | Particle Size Distribution | Particle Sizes | PCE | Permeability | Phase Diagram | Phase Separation | Physical Testing | Pinholing | Plainsman Clays | Plaster Bat | Plaster table | Plasticine | Plasticity | Plucking | Porcelain | Porcelaineous Stoneware | Pour Glazing | Powder Processing | Precipitation | Primary Clay | Primitive Firing | Propane | Propeller Mixer | Pugmill | Pyroceramics | Pyrometric Cone | Quartz Inversion | Raku | Reactive Glazes | Reduction Firing | Reduction Speckle | Refiring Ceramics | Refractory | Refractory Ceramic Coatings | Representative Sample | Respirable Crystalline Silica | Restaurant Ware | Rheology | Rutile Glaze | Salt firing | Sanitary ware | Sculpture | Secondary Clay | Shino Glazes | Shivering | Sieve | Sieve Shaker | Silica:Alumina Ratio | Silk screen printing | Sintering | Slaking | Slip Casting | Slip Trailing | Slipware | Slurry | Slurry Processing | Slurry Up | Soaking | Soluble colors | Soluble Salts | Specific gravity | Splitting | Spray Glazing | Stain Medium | Stoneware | Stull Chart | Sulfate Scum | Sulfates | Surface Area | Surface Tension | Suspension | Tapper Clay | Tenmoku | Terra Cotta | Terra Sigilatta | Test Kiln | Theoretical Material | Thermal Conductivity | Thermal shock | Thermocouple | Thixotropy | Throwing | Tony Hansen | Toxicity | Trafficking | Translucency | Transparent Glazes | Triaxial Glaze Blending | Ultimate Particles | Underglaze | Unity Formula | Upwork | Variegation | Viscosity | Vitreous | Vitrification | Volatiles | | Water in Ceramics | Water Smoking | Water Solubility | Wedging | Whiteware | Wood Ash Glaze | Wood Firing | Zero3 | Zero4 | Zeta Potential


Warping happens during the firing of ceramic ware when there is a high degree of vitrification and inadequate measures are taken during forming and firing to prevent it. Unexpected warping often happens with unstable shapes and over firing.

Key phrases linking here: warping - Learn more


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 overfired 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 densest 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. Nobody 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 superplastic 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, which means they will warp. Feldspar is the melter that makes 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 than 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, they 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.

Related Information

Why does this bowl shape always warp in the glaze firing?

Tap picture for full size
Porcelain bowl warping during firing

Here is an example of how a cast porcelain piece, L3778G, can warp during the glaze firing (the one on the left is just bisque fired, the one on the right is fired to enough maturity to achieve translucency). Several factors contribute to this failure:
1. It is cast and the walls are very thin.
2. This porcelain is highly vitreous.
3. This shape has no inherent strength to resist rim warping.
Thus the following steps will help to reduce the issue: Switching to machine forming (which orients particles concentrically), reducing the feldspar in the recipe (to reduce fired maturity) or firing lower, casting thicker walls and changing to a more flared shape. There is one other option (borrowed from bone china): Fire the piece upside down on a custom alumina setter fitted to the final rim diameter - then clear glaze it at a lower temperature.

Fired deformation comparison between two porcelains

Tap picture for full size

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.

How much porcelain flux is too much?

Tap picture for full size

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.

Fired warping can be due to shape also

Tap picture for full size

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.

Severe fired warping when a low temperature clay goes to cone 6

Tap picture for full size

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.

Fire organic ceramic shapes with this warping body for cone 6

Tap picture for full size
Two warped small square planter shapes

Left is L4410K, right is L4410L. Very similar, but L has 5% more dolomite. And these cone 6 glazes work well, G2934Y left and G2936A right. These are slip-cast pieces, the walls are not super thin, but the straight-sided shape makes them more susceptible to warping and buckling. This effect can also be achieved using talc bodies, but they have the issue of being brittle, volatile, bloating and not fitting glazes. But both of these pieces have excellent fired strength at cone 6, they have a porcelain-like surface. Of course you will need to test shapes, adjust dolomite content and control firing temperature carefully to be able to do this with consistency. If you are near a Plainsman Clays distributor, they made a test run of L4410L (as the new L213), boxes are available for order. If not, you can mix your own.

If you over fire a translucent porcelain like Polar Ice, what happens?

Tap picture for full size

Overfired Polar Ice porcelain. This bowl fired with an oval-shaped rim and was sticking to the shelf.

What happens when a terra cotta body is fired to cone 6

Tap picture for full size

It may not melt, but will certainly warp and blister/bloat (as show here). If there is inadequate kiln wash or silica sand it will also stick to the kiln shelf.

Inbound Photo Links

A 3D-printed spout enables a flared rim on cast ware

Two bars ready for pyro-plastic comparison test

A porcelain mug warps under the weight of its own handle


Glossary Bloating
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.
Glossary Maturity
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.
Troubles Body Bloating
Bloating occurs when the off-gassing of decomposing particles in a body has not completed by the onset of density and impermeability associated with the vitrification process.
Articles 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
By Tony Hansen
Follow me on

Got a Question?

Buy me a coffee and we can talk, All Rights Reserved
Privacy Policy