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Terra cotta

'Terra Cotta' (Italian for 'cooked earth') is red burning earthenware. 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. However fired terra cotta ware has a much better ability to withstand sudden temperature changes without cracking. Other attractions include dramatically lower energy costs, less kiln wear-and-tear in firing it and the ability to achieve brighter colors than at higher temperatures. If covered by a well melted, well adhered uncrazed 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' rather than 'terra cotta'. It is no accident that terra cotta fires red (although the actual raw clay can be brown, maroon, green or red). 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. Engineers refer to the degree to which a clay has been fired as its "maturity". On a scale of zero (being not fired) to ten (melted), terra cotta is about a four while translucent porcelain is around eight. Bodies that are very mature are said to be 'vitrified'. A measure of fired clay maturity is its amount of pore space (measured by noting the increase in weight of a bar after being boiled in water). Most porcelains have zero porosity while earthenwares fired to cone 04 have about 10% porosity. Terra cottas respond in a non-linear fashion to heating beyond cone 04, densifying and melting much more quickly than you would expect. Many can be warping and blistering by cone 01 and melting by cone 2. That is why cone 04-03 is seen as the sweet-spot, a compromise between strength of higher temperatures and stability of lower ones.

It is possible to create a more dense terra cotta at cone 04-03 by adding non-leaded frits (man made glass powders that melt at low temperatures). With even small additions it is impossible to vitrify a body at these temperatures (unfortunately the increased maturity turns the color to a deep solid brown instead of the traditional warm red). Being careful not to add too much frit it is possible to produce fired ware that rivals stoneware in strength. Of course, more careful control of firing and the formulation and fitting of glazes and slips is required. Rationalization is also needed to determine if the added cost of the body outweighs the reduced cost of firing.

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 clay matrix below. The clay-glaze interface is not as well developed (the glaze is not stuck on as well as stoneware). For this reason, base transparent glazes (from which colors are developed) need to be fitted to the body for the specific temperature. 'Fitted' means that they are tested to be thermal expansion compatible, they have been subjected to two tests. The first is where ware heated to 300F (for example) is plunged into ice water to see if it crazes. The second is where ware is taken from a freezer and plunged into boiling water to see if the sudden expansion of the glaze will cause it to flake off of sharp contours.

Hardness and resistance to chipping is also a matter of testing. Glazes need to be well melted and have a chemistry having as much silica and alumina as possible. 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). Notwithstanding this, people in many cultures have eaten from lead glazed earthen cookware and table ware for millenniums and continue to do so to this day. Those glazes contribute greatly to making the ware more durable.

Low temperature glazes host bright and vibrant colors, this is a key reason for the popularity of terra cotta. This is because the source of those colors, metal oxides, do not reach a sufficient temperature to boil away or vaporize. Stains (man made colorant powders) suspend in the glaze melt without melting at all and impart the maximum color they are capable of. However those colors need a white background for bright tones. That background is applied by dipping ware into a white liquid clay (called a 'slip' or 'engobe') before it has completely dried. The white slip, if paired well to the body (so it has compatible drying and firing properties) produces ware of greater utility. Potters can mix colored stain powders with the white slip and paint these on in designs after the slip has dried enough. Then the ware can be first-fired (bisque fired), a clear glaze applied and finally fired to cone 04-03 to produce a vibrant result. A variation on this in the 'majolica' process. Majolica ware is terra cotta clay which has been bisque-fired and 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.

Achieving a crystal-clear, durable, defect free, non-crazed glaze surface on terra cotta is not trivial. Often terra cotta glazes have poor working properties (they settle quickly in the bucket or turn into a jelly that is difficult to apply). Success is achieved with a combination of the right chemsitry to produce a good glaze, the right recipe and mixing and gelling procedure to produce good working properties, the right bisque temperature and application technique and the right firing schedule to melt and level it well. This is a tall order for stoneware potters who have become accustomed to working in much more forgiving circumstances. Ideally, you should buy your terra cotta clay from someone that can provide support for the entire process. Buying it as a kit with the slip, glaze, colors and instructions all included is even better. If you can make it work well you could spend a lifetime with the process!

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 an over fired middle temperature high iron raw clay (Plainsman M2). It is still stable, dense and apparently strong at cone 4 (having 1.1% porosity). But between cone 6 and 7 (top bar) it is already bloating badly. Such clays must be fired at low enough temperatures to avoid this volatility (if accidentally over fired). This clay only reaches a minimum of 1% porosity (between cone 4 and 5), it is not possible to fire it to zero porosity. This is because of the particulate gas-producing particles (it is ground to 42 mesh only).

What happens when you fired a terra cotta at cone 5-8?

This one can take more temperature than most. It looks OK at cone 5 (bottom bar). But at cone 6 bloating (bubbles) begin to occur. This body, while smooth to the touch, contains some iron and sulphate particulates that generate gases during firing, these are the catalyst for the bloating (the clay matrix becomes dense enough that it can no longer vent the gases of decomposition through it).

What material makes the tiny bubbles? The big bubbles?

These are two 10 gram balls of Worthington Clear glaze fired at cone 03 on terra cotta tiles (55 Gerstley Borate, 30 kaolin, 20 silica). On the left it contains raw kaolin, on the right calcined kaolin. The clouds of finer bubbles (on the left) are gone from the glaze on the right. That means the kaolin is generating them and the Gerstley Borate the larger bubbles. These are a bane of the terra cotta process. One secret of getting more transparent glazes is to fire to temperature and soak only long enough to even out the temperature, then drop 100F and soak there (I hold it half an hour).

How bad can efflorescence of soluble salts be?

Like this! This terra cotta clay vitrifies here at 1957F (cone 03). 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.

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 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.

Some iron clays bloat before reaching zero porosity, others do not

A very fine particled low fire red burning terra cotta clay (Plainsman Redearth) fired at cone 2,3 and 4 (top to bottom). Notice the cone 4 bar is beginning the melting process (signaled by the fact that it is expanding). Yet it is not bloating as this type of raw clay normally would. The cone 2 and three bars have reached zero porosity also. Other clays that fire to very similar color begin to bloat long before they reach zero 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.

Does this terra cotta clay have an LOI higher than kaolin? No.

These two samples demonstrate how different the LOI can be between different clays. The top one is mainly Redart (with a little bentonite and frit), it loses only 4% of its weight when fired to cone 02. The bottom one is New Zealand kaolin, it loses 14% when fired to the same temperature! The top one is vitrified, the bottom one will not vitrify for another 15 cones.

Cone 04 terra cotta cross section close-up with glaze

The glaze is well melted, but the interfacial zone with the body is very narrow. It is basically just stuck on the surface. The body is not developing any clearly visible glassy phases as does porcelain and stoneware, so not surprisingly, its strength is much lower than vitrified clay bodies at higher temperatures. However it is possible to add a frit and glass-bond the particles at cone 02 (at much higher cost of course). Not surprisingly, glazes must be more closely tuned to match the thermal expansion of the body for lower temperatures (since they are not stuck on as well).

Clay from below a bog, ready to throw right out of the ground!

This little pot on the left is more than it appears. Both of these samples were fired at cone 01 and clear glazed in the green state (without bisquing). The one on the right, a typical unprocessed native terra cotta clay, is full of pinholes, the one on the left has none. But the one on the left has been pre-processed by mother nature: it is from a thick layer of clay found below a bog in northern Alberta. It has 21% water content, you just cut a piece out, wedge it and throw a pot! Although it contains some particulate, it is highly plastic yet dries well and fires to a dense stoneware-like hardness 11 cones lower than we normally make stoneware at.

Double-slip layer incised decoration: A challenge in slip-body fitting

An example of a white engobe (L3685T) applied over a red clay body (L3724F), then a red engobe (also L3724F) applied over the white. The incised design reveals the white inter-layer. This is a tricky procedure, you have to make sure the two slips are well fitted to the body (and each other), having a compatible drying shrinkage, firing shrinkage, thermal expansion and quartz inversion behavior. This is much more complex that for glazes, they have no firing shrinkage and drying shrinkage only needs to be low enough for bisque application. Glazes also do not have quartz inversion issues.

Terra cotta and a surprising thing about thermal shock

This terra cotta cup is glazed with G2931G clear glaze (Ulexite based) and fired at cone 03. It survives 25 seconds under direct flame against the sidewall before a crack occurs. Typical porcelains and stonewares would survive 10 seconds. Super vitreous porcelains 5 seconds. This is a key advantage of earthenware. Sudden changes in temperature cause localized thermal expansion, this produces tension and compression that easily cracks most ceramics. But the porous nature of earthenware absorbs it much better. During initial testing I found better performance for glazed earthenware (vs. unglazed), but in later testing they proved to be fairly similar.

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).

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.

I want this engobe to gel in ten seconds. Why?

It is going to be applied to leather hard earthenware and it needs to be thixotropic. Why? I do not want it to run down from the rims of the mugs after dipping. The process: Stir the engobe, pour-fill the mug, pour it out and push it upside down into the engobe. If I can pull it back out before the 10 seconds is up I get a perfectly even gelled layer that does not move. A good test is to stir it then pull out the spatula slowly. If it hangs on in a even layer with only a few drips it is perfect. I also I tip the measuring cup to check it is gelling. Achieving this behaviour requires very careful additions of epsom salts (and thorough mixing between). As the slip approaches this 10 second threshold even a slight bit more salts will turn it into a bucket of jelly! I almost always overdo it! So I keep some thinner slip aside to dilute it down to perfect, not much is usually needed.

Underglazes, engobe, a good transparent glaze and cone 03. Life is good!

The white engobe was applied by pouring at leather hard stage. The underglazes were also painted on at leather hard. The mugs were then dried, cleaned, bisque fired, dipping in clear glaze and final fired to cone 03. The clay and engobe have frit additions to make them vitrify at low temperatures.

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

Terra cotta bodies are more volatile, maturing more rapidly over a narrower range than others. These bars are fired (bottom to top) at cone 06, 04, 03, 02, 2 and 4. This is Plainsman BGP.

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

It may not melt, but will certainly warp and blister/bloat. If there is inadequate kiln wash it will stick to the kiln shelf.

Can terra cotta ware resist an open flame? Yes.

This is a road-side stand in Mexico in 2016. Each of these "cazuelas" (casseroles) have a 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 and a very thin layer of lead glaze is painted over (producing a terra sigilatta type appearance, but with brush stoke texture). These have been made and used here for hundreds years. How can they not crack over an open flame? The flame is small. The clay is fired as low or lower than potters in Canada or the US would even fired their bisque. It is porous, open and able to absorb the stresses. They know these pieces are not strong, so they treat them with care.

Surface tension is a big deal in transparent glazes at cone 04

Low fire glazes must be able to pass the bubbles their bodies generate (or clouds of micro-bubbles will turn them white). This cone 04 flow tester makes it clear that although 3825B has a higher melt fluidity (it has flowed off onto the tile, A has not). And it has a much higher surface tension. How do I know that? The flow meets the runway at a perpendicular angle (even less), it is long and narrow and it is white (full of entrained micro-bubbles). Notice that A 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 similar to SG13, although flowing less). However they all dry very slowly. Watch for a post on G2931J, a Ulexite/Frit-based recipe that works like A but dries on dipped ware in seconds (rather than minutes).

Particle size and LOI determine behaviour of over-fired bodies

These are four terra cotta body disks that have been fired to cone 10 reduction. The fluxing action of the iron has assisted to take them well along in melting. Notice that one is hardly bubbling at all, it is Redart clay that has been ground to 200 mesh (the lower right one is a body mix of 200 mesh materials also containing it). The upper left one is bubbling alot more. Why? Not just because it is melted more (in fact, the one on the lower left is the most melted). It is a body made from clays that have been ground to 42 mesh. Among the particles are larger ones that generate gases as they decompose. Yes, the particles in the others do the same, but their smaller size enables earlier decomposition and expulsion of smaller gas amounts distributed at many more vents. Some bodies cannot be fired to a point of zero porosity, they will bubble before they get there.

Terra cotta vs. Terra cotta stoneware

These terra cotta mugs are fired at cone 03. Although the glaze on the left one is melted well the terra cotta itself has a porosity of more than 10%. The mug on the right is a finer grained terra cotta with added frit to make it vitrify. It is thus dramatically stronger and more durable, rivalling high temperature stoneware. Neither of the glazes are crazed, but the glaze on the right is much more firmly attached and resistant to future crazing. Does the mug on the left have an advantage? Yes. Although both can withstand hot coffee being poured in, the one on the left can withstand more dramatic thermal shocks without the piece itself cracking.

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By Tony Hansen

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