All Glossary

Terra Cotta

A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.

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'Terra Cotta' (Italian for 'cooked earth') is red-burning earthenware ceramic. Structural terra cotta products are most familiar (e.g. brick, pipe, tile), however, this page is about terra cotta pottery, it has been made for thousands of years by indigenous cultures, most often unglazed. If glazed, high-lead-content mixtures have been traditional. It is fired at much lower temperatures than stoneware so, not surprisingly, it is not nearly as strong and durable. However, fired terra cotta ware has a better ability to withstand sudden temperature changes without cracking. Other attractions include dramatically lower energy costs, less kiln wear-and-tear, the ability to achieve brighter colors and the ability to fire ware on stilts (and thus have glazed bottoms). If covered by a well-melted, well-adhered and well-fitted glaze, terra cotta can provide excellent service for functional ware.

Terracotta is often used in industry to make sculptures, tile, planters, garden and architectural ware. When it is glazed, ware is often referred to as 'red earthenware'. Although Terra cotta fires red the actual raw clay can be brown, maroon, green or red (because of high iron oxide content). Red-burning low-fire clays are available almost everywhere. They are almost always very impure, contaminated by a range of fluxing-bearing minerals and iron oxide (which produces the red fired color). The contaminants act as fluxes, making the material fire much harder than would a mix of white burning clays at the same temperature. That being said, terra cotta clays do not develop fired maturity in their normal firing range of cone 06-04. Most stonewares, for example, have less than 2% porosity while earthenwares fired to cone 04 have about 10-12%. Terra cottas respond in a non-linear fashion to heating beyond cone 04, densifying and melting increasingly rapidly often warping and blistering by cone 01 and melting by cone 2 (technically this is referred to as "a narrow range between the points of incipient fusion and viscosity"). Cone 04 is seen as the sweet-spot, a compromise between strength of higher temperatures and stability and red color of lower ones.

It is possible to create a more dense terra cotta at cone 04-03 by adding even small percentages of body frit (e.g. Ferro frit 3110 as used in Zero3 stoneware). Unfortunately, the increased maturity darkens the color to a deep solid brown and requires refitting of glazes and slips. And it can bubble boron blazes. One real benefit is that the frit increases thermal expansion so glazes will be less likely to craze.

Because terra cotta ware is porous it is very important that the glaze and body thermal expansions match (called "fit"). If they do not, craze lines (cracks in the glaze itself) will develop and enable water to penetrate and absorb into the porous clay matrix below. The clay-glaze interface is typically well developed. For this reason, base transparent glazes (from which colors are developed) need to be fitted to the body for the specific temperature and they need to fire crystal clear. 'Fitted' means that they are tested to be thermal expansion compatible. For example, tendency to craze or shiver can be tested using the IWCT test or BWIW test. Lead glazes perform exceptionally well at low temperatures (and are inexpensive), but are not common in developed countries (where boron is used instead as a melter). A wide variety of borax frits are available and service well down to cone 06 to produce ultra-transparent glazes.

Low-temperature glazes and underglaze colors are bright and vibrant (from metal oxides and stains), this is one key reason for the popularity of terra cotta. However, those colors are best over a white background. An engobe paired well to the body and applied at leather-hard stage provides that. Then ware can be bisque fired and a clear glaze applied. A variation on this is the 'majolica' process. Majolica ware is terra cotta clay that has been bisque-fired and thickly covered with an opaque white glaze. That becomes the base for painting on metal-oxide stained overglaze colors. This process has the disadvantage of putting the metal oxide into contact with food, whereas the colored slip process isolates them behind a transparent glaze.

Many potters working at high temperatures have eyed the terra cotta process with some envy. But wrong assumptions bred by stoneware experience can sour attempts at adopting it. Crystal clear transparent glazes are an obvious expectation but they are deceptively difficult to achieve. In the terra cotta process many mineral particles are decomposing and expelling significant gasses in the process, this spawns micro bubble clouding. To make matters worse, high boron tends to create clouding yet the boron is needed to melt the glaze. Several strategies help.

1. Ware is typically bisque fired to a higher temperature than the glaze firing. Of course, the bisque ware still needs to be porous so bisque is generally no hotter than cone 04. That means glazes are fired from 06-04 creating “a playground” of 120 degrees F with minimal LOI gasses.

2. Firing schedules are another key to success. An excellent strategy to minimize surface bubbles and issues is to employ a drop-and-hold, such as the 04DSDH schedule.

3. A good glaze recipe is very important (we recommend the G1916Q transparent base with optional iron addition). The G3879 is even better if you can get the frits.

4. Use a quality, fine particled red burning terra cotta clay, even mixing your own if needed (e.g. the L4170B recipe).

Related Information

A terra cotta clay fired from cone 06 (bottom) to 4

Terra cotta bodies are more volatile in the kiln than stonewares. They mature rapidly over a narrower range of temperatures, that process is accompanied by dramatic changes in fired color, density and fired strength. These bars are fired (bottom to top) at cone 06, 04, 03, 02, 2 and 4. This is Plainsman BGP (a quarry material), cone 02 finds it at maximum density (and fired shrinkage). At cone 06 (1830F/1000C) it is porous and shrinks very little. But as it approaches and passes cone 03 (1950F/1070C) the color deepens and then moves toward brown at cone 02 (where it reaches maximum density and stoneware strength). However, past cone 02 it becomes unstable, beginning to melt (as indicated by negative shrinkage). This is typical of most terra cotta clay materials. The good news: With careful selection of temperature this clay can produce strong and durable pottery at a very low, and economical, temperature.

How to decide what temperature to fire this terra cotta at?

Let's suppose you need strength and density for utilitarian ware. These SHAB test bars characterize a terra cotta body, L4170B. While it has a wide firing range its "practical firing window" is much narrower than these fired bars and graph suggest. On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But ware will warp during firing and transparent glazes will be completely clouded with bubbles (when pieces are glazed inside and out). What about cone 3? Its numbers put it in stoneware territory, watertight. But decomposition gases still bubble glazes! Cone 2? Much better, it has below 4% porosity (any fitted glaze will make it water-tight), below 6% fired shrinkage, still very strong. But there are still issues: Accidental overfiring drastically darkens the color. Low-fire commercial glazes may not work at cone 2. How about cone 02? This is a sweet spot. This body has only 6% porosity (compared to the 11% of cone 04). Most low-fire cone 06-04 glazes are still fine at cone 02. And glaze bubble-clouding is minimal. What if you must fire this at cone 04? Pieces will be "sponges" with 11% porosity, shrinking only 2% (for low density, poor strength). There is another advantage of firing as high as possible: Glazes and engobes bond better. As an example of a low-fire transparent base that works fine on this up to cone 2: G1916Q.

Can terra cotta ware survive an open flame without cracking?

It can if the flame is not too big! This is a roadside stand in Mexico in 2016. Each of these "cazuelas" (casseroles) have a small flame under them to keep the food inside warm. The pedestal is unglazed. The ware is thick and heavy. The casseroles are hand decorated with under-glaze slip colors with a very thin layer of lead glaze painted over (producing a terra sigilatta type appearance, but with brush stroke texture). These have been made and used here for hundreds of years. How can they not crack over an open flame? The flame is small and it is applied in the center of the bottom so that stresses are distributed radially and symmetrically. The ware is fired at what potters in Canada or the US would consider bisque temperature. It is porous, open and able to absorb the stresses. Some of them do have small cracks, but these seem to relieve stresses and prevent more. They know these pieces are not strong, so they treat them with care in heating, handling and washing. They know the glaze will leach lead or even dissolve if they put acidic liquids in them - so they don't do that.

What temperature is Mexican terra cotta pottery fired to?

This piece was made in Puebla, Mexico by a potter using traditional techniques and local clays. I have sawed it in half and refired one half at 1850F (Orton cone 04). That half (the right one) has not shrunk, which means it has not exceeded the temperature of the original firing (the piece was likely glazed in the dry state, not bisque fired). Notice how the refire has darkened the body color. This is probably happening because the lead glaze is maturing the surface of the clay (without the glaze it would take a much higher temperature to darken it to this extent, and, that would also entail shrinkage).

Craft store selling traditional terra cotta ware in Mexico 2020

This ware is used all over the city by street-side restaurants and food vendors. And routinely used in the house. It is all lead-glazed. They all know to handle it with care to minimize breakage. Surfaces and edges are rough, it is poorly finished but most people value the tradition enough to not even notice. Of course factory-made ware is much stronger and more functional, and cheaper. But at meals and occasions many seek opportunties to use this at the table.

An incredibly white engobe on terra cotta that produces durable and decoratable low fire ware

I found seven secrets with recipe, process, glaze and firing to make durable terra cotta.
1. A lot of Zircopax, in this case 20% (for whiteness, opacity).
2. The whitest burning materials: New Zealand Halloysite as the kaolin and nepheline syenite as the feldspar.
3. 3% Veegum to gel the slurry (enabling low specific gravity for thin and even coating).
4. The recipe, L3685Z2, has 55% kaolin, that will certainly produce drying cracks. But 1% CMC gum stops that and makes it brushable. It even works on on bisque, I pour-applied it to the insides of these two slip-cast pieces, it drained to perfectly even coverage (in a very thin layer).
5. A terra cotta casting/throwing body to fit the engobe to (has the same fired shrinkage at a target temperature, e.g. cone 04): Initially I am using L4170B.
6. A clear glaze that fits and is transparent: Notice how much whiter the left one is, G3879. At the same thickness as the G1916Q on the right, it is more transparent, better transmitting the white of the engobe.
7. The right firing curve: The 04DSDH drop-and-hold schedule for defect free surfaces.

Transparent glaze quality on terra cotta is about multiple things

Glaze chemistry, slurry preparation and application thickness. These two mugs are made from the same clay, Plainsman Snow, and Amaco V-303 Terra Cotta underglaze has been applied to each. The dipping glaze on the left was mixed from a recipe, slurried and then sieved at 80 mesh. However the sieve did not break up the agglomerates of Veegum and New Zealand kaolin. This problem can be quickly resolved by mixing batches of the glaze in a kitchen blender at top speed. The glaze on the right is Spectrum 700, it has been well mixed and sieved. The second issue is thickness: The mug on the left was dipped and held in too long, getting it on far too thick. On the right, the gummed glaze was painted on, giving better control to limit thickness (three thin coats were applied).

G1916Q transparent on terra cotta body at cone 06, 05, 03

The body is Plainsman L215. We used the 04DSDH firing schedule. The glaze is inexpensive to make so we have a 2 gallon bucket. It has good dipping much like a stoneware glaze so it is easy to apply quickly and evenly. For most terra cottas, body strength increases dramatically by cone 03. However the most transparent and glassy glaze surface happens at cone 06. Terra cotta bodies need to be bisque fired fairly low (e.g. cone 06) to have enough porosity to work well with dipping glazes. After cone 06 they generate increasing amounts of gases (as various particle species decompose within), for this reason the glazes can have more micro-bubble clouding or tiny dimples in the surface. This glaze has 2% iron oxide added as a fining agent to remove the bubbles. That iron also reddens the color and variegates the surface somewhat. Even though the surface character at cone 03 is not a smooth, it has a natural charm, and the color is very rich. And that piece has stoneware durability and strength.

Iron oxide as a fining agent to debubble a low fire transparent

These terra cotta clays were bisque fired at cone 04 and glaze fired to 04 using the 04DSDH schedule. The glaze is G1916Q, an expansion-adjustable cone 04 clear. That schedule alone is often enough to get transparent, defect free glazes in many situations. But not in this case. The solution was to add a fining agent. In this case we added 2% red iron oxide (to the top glaze). The particles of iron floating in the melt acted as a congregating points for bubbles, helping them to escape. And we got a bonus: a more interesting aesthetic. A 1% addition also worked, but not as well (we have settled on 2% iron and screening the glazes to 100 mesh). Screening out the larger particles slightly degraded the fining performance (so we have to accept the tiny specks). Iron does not always workin other situations. Other fining agents we have used at cone 6 do not work in this situation (e.g. 2% Zircopax, Alumina). Of course, this glaze will fire amber on a white body.

Two transparents having opposite melt fluidity/surface tension balances

Low-fire glazes must be able to pass the bubbles they and the underlying bodies generate (or clouds of micro-bubbles will turn them white). This cone 04 flow tester makes it evident that 3825B has a higher melt fluidity (A has not even dripped onto the tile). And its higher surface tension is demonstrated by how the flow meets the runway at a perpendicular angle (it is also full of entrained micro-bubbles). Notice that A, by contrast, meanders down the runway, a broad, flat and relatively clear river. Low-fire glazes must pass many more bubbles than their high-temperature counterparts, the low surface tension of A aids that. A is Amaco LG-10. B is Crysanthos SG213 (Spectrum 700 behaves similarly, although flowing less). These two represent very different chemistry approaches to making a clear glaze. Which is better? Both have advantages and disadvantages.

Match the firing shrinkage of an engobe to a terra cotta

These lines plot the firing shrinkages for three versions of L3685Z2 engobe. Each line was derived by doing a multi-temperature SHAB test. Notice the terra cotta body I want to match (red line) and the black engobe (green line) do not cross anywhere. That means there is no temperature at which they fit each other (the engobe always has 2% or more firing shrinkage). Notice the L4170B terra cotta fits the white version, Z2, at 2150F (red line crosses blue line, but the body is over-fired by that point). For a fit at my preferred 2000F (cone 02) I need the Z4 engobe to shrink 2% more (a 3% addition of frit 3110 will do that). What about the black Z4? That is the opposite situation, it already contains 5% frit, removing that will drop that green line about 3%, hitting the red line at 2000F (and following it all the way down past 1950 into the cone 04 range). I ignored all of this and used the Z2 white on L4170B, L210 and L215. It looked good on most pieces, but sure enough, it did crack around abrupt contours on some. Of course, this does not assume a thermal-expansion-match of body and engobe.

Why does this terra cotta clay bubble glazes?

But this does not happen when a piece is glazed only on the inside or outside. Why? As terra cottas approach vitrification they generate gases as particles within decompose. At cone 04 it is not an issue, transparent glazes will fire ultra-clear and defect-free. But fire that same body to cone 02 or 01 and the glaze can be filled with bubbles and surface blisters. If gases have no other way to escape the body except by bubbling up through the glaze, this is what happens. But, where adjacent unglazed surfaces exist, all the gases will route out through it, leaving the glazed one defect free. Of course that is not practical here. So the only solution is to fire lower. If vitrification is really needed then a frit must be added to the body to densify it at that lower temperature.

Cross section comparison between cone 04 terra cotta and cone 6 iron stoneware

Both of these bodies have a range of particle sizes (from 42 mesh and finer). The glaze on the cone 04 section (close-up on the left) is very well melted, but its interfacial zone with the body is narrow, it is basically just "stuck on" the surface (so it must be tuned to match the thermal expansion of the body to prevent crazing/shivering). The body is not developing any clearly visible glassy phases, meaning that any strength it has is purely a product of sintering. On the right, the interfacial zone of glaze-with-stoneware-body is wider. The body has incipient melting (tiny feldspar particles are fusing and beginning to dissolve into the surfaces of surrounding ones). The color has changed from red to brown.

How bad can efflorescence of soluble salts on fired ceramic be?

Like this! This terra cotta clay matures to good strength around 1950F. Notice how the salts have concentrated on the outer and most visible surface, the more vitreous this fires the worse it looks! The piece was dried upside down so of course, all the water had to escape through that route. A complicating factor is how handling of the piece at the leather hard stage has made it even more unsightly. This problem is common in many terra cotta materials but can also surface in others. Barium carbonate can be used to precipitate the salts inside the clay matrix so they do not come to the surface on drying. There is good news: Solubles salt deposition can actually be much worse than you see here.

Test bars of different terra cotta clays fired at different temperatures

Bottom: cone 2, next up: cone 02, next up: cone 04. You can see varying levels of maturity (or vitrification). It is common for terra cotta clays to fire like this, from a light red at cone 06 and then darkening progressively as the temperature rises. Typical materials develop deep red color around cone 02 and then turn brown and begin to expand as the temperature continues to rise past that (the bottom bar appears stable but it has expanded alot, this is a precursor to looming rapid melting). The top disk is a cone 10R clay. It shares an attribute with the cone 02 terra cotta. Its variegated brown and red coloration actually depends on it not being mature, having a 4-5% porosity. If it were fired higher it would turn solid chocolate brown like the over-fired terra cotta at the bottom.

Some bodies cannot be fired to even near zero porosity

Bloating in a high iron raw clay ground to 42 mesh (Plainsman M2). It is still stable, dense and apparently strong at cone 4 (having 1.1% porosity). But at cone 6 (top bar) it is bloating badly. At cone 5 the clay experiences the early stages of bloating. Cone 4 is thus "dangerous territory" for this particular clay. A reminder of this can be seen by putting on a transparent glaze - it fills with clouds of micro-bubbles from off-gassing that has begun well below cone 4.

Cone 2: Where we see the real difference between terra cottas and white bodies

The terra cotta (red earthenware) body on the upper left is melting, it is way past zero porosity, past vitrified. The red one below it and third one down on the right have 1% porosity (like a stoneware), they are still fairly stable at cone 2. The two at the bottom have higher iron contents and are also 1% porosity. By contrast the buff and white bodies have 10%+ porosities. Terra cotta bodies do not just have high iron content to fire them red, they also have high flux content (e.g. sodium and potassium bearing minerals) that vitrifies them at low temperatures. White burning bodies are white because they are more pure (not only lacking the iron but also the fluxes). The upper right? Barnard slip. It has really high iron but has less fluxes than the terra cottas (having about 3% porosity).

Terra cotta clays are not created equal

A variety of terra cotta clay test bars (and a low temperature porcelain) that have been fired to cone 5. The measurement and weight data from these bars is entered into the appropriate recipes in my account at Insight-live.com; it uses that data to calculate shrinkage and porosities. I will also attach link this picture to each of the recipes. Some are quite vitreous and stoneware-like, some are in the advanced stages of melting, others could take more heat yet.

Red burning, customer-found terra cotta clays charachterized

We tested four different clays (brought in by customers). One is from BC and three from Alberta. These fired sample bars show rich color, low soluble salts and high density and strength at very low temperatures. L4233 (left): Cone 06 to 3 (bottom to top). Reaches stoneware-density at cone 02 (middle bar). Plasticity is very low (drying shrinkage is only 4.5%). But, it is stable even if over-fired. L4254 (center bottom): Cone 04,02,3,4 (bottom to top). Very fine particled but contains an organic that is gassing and bloating the middle two bars. L4243: Fires lighter and looks stable here (cone 02,01,1,2 shown) but melts suddenly less than a cone above the top bar (well before vitrification is reached). L4242 (right): Hyper-plastic, with 12% drying shrinkage! Already melting by cone 02 (third from top). Achieves almost zero porosity (porcelain density) at cone 04 (#2 bar). Even when mixed with 20% kaolin and 20% silica it still hits zero porosity by cone 1. What next? I'll mix L4233 (left) and L4242 (right), that should produce stoneware density at cone 02 (about 1% porosity).

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

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.

The porcelain is harder, but the terra cotta has it beat for thermal shock!

This terra cotta cup (center) is glazed with G2931G clear glaze (Ulexite based) and fired at cone 03. It survives 30 seconds under direct flame against the sidewall and turns red-hot before a fracture occurs (the unglazed one also survived 30 seconds, it only cracked, it did not fracture). The porcelain mug (Plainsman M370) is glazed with G2926B clear, it survived 15 seconds (even though it is much thinner). The porcelain is much more dense and durable, but the porous nature of the earthenware clearly withstands thermal shock much better. It is actually surprisingly durable.

Terra cotta gets brittle when over fired

An excellent example that demonstrates the brittleness typical of vitrified terra cotta bodies. This bowl was fired to cone 02 and rung like a tuning fork when struck with a spoon. The body is dense like a porcelain and at appeared to be incredibly strong (this body is much more vitreous than an average terra cotta would be). However after a few more taps with the spoon it broke in two! It is brittle! Very hard, but brittle. At first I thought it might be that the glaze is under compression, but when I dropped the halves they did not shatter in the manner characteristic of compressed glaze, and they broke with razor sharp edges (like a vitrified porcelain does). So firing for this body must stop short of the most dense matrix possible to avoid this brittleness.

Creating a Body/slip Equilibrium in Terra Cotta Ware

L3685X white slip (left mug) has 5% more frit than Y (right). The frit is a melter, creating more glass bonds to adhere it to the body (it also hardens it and darkens the color a little). But the frit also increases firing shrinkage, 'stretching' the white layer on the body as the kiln cools (the slightly curled bi-clay bar demonstrates that). However the glaze, G2931G, is under some compression (to prevent crazing), it is therefore 'pushing back' on the white slip. This creates a state of equilibrium. The Y slip on the right is outside the equilibrium, it flakes off at the rim because the bond is not good enough. Adding more frit, the other side of the balance, would put the slip under excessive tension, reducing ware strength and increasing failure on exposure to thermal shock (the very curled bi-clay bar in the front, not this clay/slip demonstrates the tension a poorly fitted slip could impose).

What does it take to opacify a low temperature terra cotta glaze

Tin oxide is a powerful opacifier, but the 5% in the glaze on the left is clearly not enough. 10% more zircon had to be added to produce the one on the right.

Perfect storm to create massive bubbling in a low fire kiln

The clay is terra cotta. It is volatile between 04 and 02, becoming dramatically more dense and darker colored. The electronic controller was set for cone 03, but it has fired flat (right-most cone), that means the kiln went to 02 or higher. At 02 the body has around 1% porosity, pushing into the range where the terra cotta begins to decompose (which means lots of gas expulsion). The outsides of some pieces blistered badly while the insides were perfect (because they got the extra radiant heat). The firing was slow, seven hours. Too long, better to make sure ware is dry and fire fast. Another problem: The potter was in a rush and the pieces were not dry. These factors taken together spelled disaster.

Refiring a terra cotta mug that had already bubbled only made blistering worse

Plus the glaze ran even more. The main problem was that the original firing was taken too high, about cone 02 (seven hour schedule). This body nears zero porosity there and is beginning to decompose. That generates gases. The second firing was taken to cone 03 in four hours. But the glaze just percolated more. However freshly glazed bisque ware in that same firing came out perfect. Lessons were learned. Fire faster. Keep it cone 03 or lower. Do not put the glaze on too thick. Use self-supporting cones to verify the electronic controller, they are much more accurate than regular cones.

A terra cotta bowl made in Turkey

Turkey has milleniums of experience with terra cotta. This piece uses an engobe covering on both the outside and inside. And the Turks have developed the technology to process the clays enough that they can be fired to almost porcelain strength (although this piece is low fired and porous). This piece was given to me by a resident artist in the studio at the Kale Seramik factory in Çan, Turkey (around 2002). At the time it was the biggest ceramic factory complex in the world, making everything from fine china to bricks. Many potters do not really understand what terra cotta is.

Inbound Photo Links

Cutting a terra cotta dish in half with a diamond saw from Amazon

Problem at cone 1

Two low fire transparent highly fritted glaze recipes for pottery

The glaze firing reveals that the specific gravity of this V-303 terra cotta under glaze is too low

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