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Terra cotta

The term Terra Cotta can refer to a process or a kind of clay. Terra cotta clays are high in iron and available almost everywhere. While they vitrify at low temperatures, they are typically fired much lower than that and covered with colorful glazes.

Details

'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 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 in firing it, the ability to achieve brighter colors than at higher temperatures 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' 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". 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 10-12% porosity. Terra cottas respond in a non-linear fashion to heating beyond cone 04, densifying and melting increasingly rapidly to warping and blistering by cone 01 and melting by cone 2. 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 (e.g. Zero stoneware). With even small additions it is possible 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), producing 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 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. 'Fitted' means that they are tested to be thermal expansion compatible. For example, tendency to craze can be tested using the IWCT test. Or tendency to shiver at edges can be tested by plunging chilled ware into boiling water. 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). A wide variety of borax frits are available and service well down to cone 06 to produce ultra-transparent glazes.

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 decompose 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 often applied by dipping ware into a white liquid clay (an 'engobe') at leather hard stage. The white (or colored) slip, must be paired well to the body (so it has compatible drying and firing properties). Then the ware can be bisque fired, a clear glaze applied and glaze fired, typically to cone 06-04, to produce a vibrant result. A variation on this is the 'majolica' process. Majolica ware is terra cotta clay which 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 that have worked at high temperatures have eyed the terra cotta process with some envy, not just for the energy savings, but for the traditional, warm red colors through brilliant transparent glaze surfaces. Of course, they have little desire to fiddle with paint-on glazes, so they try to develop a glaze dipping process akin to their experience with stoneware. But repeated failures sour them on the process. What is happening? The reason for failure is wrong assumptions bred by stoneware experience. Terra cotta is about awareness of a key concept: Decomposition. The terra cotta process is much more 'active', mineral particles are decomposing (expelling significant gases and imposing rapid color and maturity changes) during the process. Another difference: Low fire cones span a much wider range than high temperature ones (cone 06-02 spans 200 degrees F!). As a strategy to avoid gases having to bubble through glazes and either clouding them or leaving surface defects, ware is typically bisque fired to a higher temperature than the glaze firing. For commercial glazes that are painted on, this is practical, hobbyists are generally patient regarding the drying time between coats. But when a potter wants to mix buckets of glaze and dip ware, adequate porosity is needed to get even coverage and quick drying of a single glaze coat. That means the bisque firing temperature must be lower, cone 06 or less. A potter wants the most strength possible, so is going to want to fire at least cone 04. That is 120 degrees F higher than cone 06. Also 120 degrees of decomposition gas bubbles that need to pass the melting glazes. The result is typically cloudy glazes that hide the red color and surface defects (e.g. pinholes, blisters, dimples). Different firings produce different results and it becomes quickly discouraging. While glaze recipes are very important (we recommend G1916Q or G2931K), firing schedules are the key to success. An excellent strategy to minimize surface issues is to employ a drop-and-hold firing, such as the 04DSDH schedule. And another key factor: Accept a compromise between getting significantly more strength at cone 03 to the best glaze surface at cone 06. You may find cone 04 to be a sweet-spot. Another helpful thing is to use a quality, fine particled red burning terra cotta clay if you mix your own clay body (e.g. the Zero3 recipe makes stoneware possible). Another idea: Add 1-2% iron oxide to the clear glaze, the iron particles vacuum out the micro-bubbles. And enhance the red terra cotta color.

Related Information

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.

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. They all know to handle it with care to prevent breakage.

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

Fired bars of 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.

Terra cotta vs. low fire red 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 (has a better interface with the glaze) and therefore 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.

What happens when you fire 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).

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).

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

Three clear-glaze terra cotta mugs with rich red color

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.

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).

When it makes total sense to fire hotter than cone 04

Two mugs, the one fired at cone 03 is better.

The body is Plainsman L215 (bisque fired at cone 06). These commercial underglazes are not temperature-specific, they work the same across a wide range of temperatures. But the glaze is G2931K, it was designed for cone 03 and does not melt as well at cone 04 (for example, it goes milky if applied too thick), it was for use on Zero3 stoneware and Zero3 porcelain. This body is, however, a standard porous earthenware. While it is normally fired at cone 06-04, it tolerates cone 03. Actually, it benefits from cone 03, firing stronger and a deeper red color. So, since this glaze and body are both better at cone 03, it makes total sense to fire pieces like that at that temperature.

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.

Why this transparently-glazed terra cotta is better at cone 03 than 04

Two terra cotta mugs, the glaze on one is more glossy and the body color is richer

Terra cotta bodies are typically fired between cone 06 and 04. That being said, many, like this Plainsman L215, develop richer color at cone 03 and fire much stronger. Glazes, of course, melt better and micro-bubbles pass through easier at cone 03. But this happens only if the body has not begun to decompose (and therefore generate a lot more gases of decomposition). Notice that crazing is beginning on the one of the left. Apparently the improved body:glaze interface and the development of better vitrification reduces the problem. Cone 03 is somewhat of a sweet-spot for this specific body, however firing higher begins decomposition processes that generate gases that disrupt the surface. Needless to say, accurate firing is needed to fire at cone 03 for ongoing success (because cone 02 is too high for this body, glaze will blister). Do you know what terra cotta actually is, if not click the link to learn more (this is a big topic).

G1916Q glaze fired at cone 03 and 04 on a terra cotta clay body

Two clear glazed terra cotta mugs

The body is Plainsman L215. Both were thinly applied and fired using the 04DSDH schedule. The glaze has 2% iron oxide added and sieved to 80 mesh, this reddens the color and acts as a fining agent to reduce micro-bubble population. The one fired to cone 03 (left) is considerably stronger, better surviving the stress of successive impacts with a hammer. However, it has minute dimples in the surface, likely because it is having to clear bubbles originating from decompositions occurring in the body below (as it fires well above bisque temperature). The mug on the right fired about 40F cooler, between cone 04 and 05, only slightly above bisque. The glaze surface is much better, almost crystal clear. Since the glaze fits well the mug has surprising strength, much better than a stoneware piece with a poorly fitted glaze that shatters with one tap of a hammer. This one survived about ten whacks before a piece broke out! A big advantage of cone 04 and cooler is that ware can be fired on stilts, meaning you can glaze the whole thing, no bare clay is exposed. Again, I can only achieve this kind of glaze surface using the above-mentioned firing schedule.

How bad can efflorescence of soluble salts of fired ceramic surfaces 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.

Milk as a glaze! How is that possible?

Clay tiles, one unglazed, one carbon glazed

After watching a youtube video (link below) about a Karelian potter, who uses this technique to make cookware, I could not wait to try it. He unloads the ware from his kiln (which appears to be a standard electric top loader used by potters in the west), and while still hot he immerses pieces in a bucket of milk for a few seconds. When he withdraws them they are steaming. I mixed some 2% milk and cream (to get closer to the whole milk he was using) and cold-dipped an 1850F bisque-fired jar and tile (of Plainsman L210) for about a minute (to enable it to soak in as much as possible). The potter claims to fire his ware to 300-350 degrees. I fired 500F/hr to 612F (350C), then held for 10 minutes and shut off to free fall. And it worked beautifully, high enough to get lots of carbon (which is only on the surface), not high enough to burn it away. The surface is smooth and pleasant-to-touch, it is odor-free. The potter claims it retains this surface over many years despite repeated oven use. This clay body, L210, is well suited since it is very fine-grained and fires to such a smooth unglazed surface. And the carbon makes it much better. Indigenous cultures throughout history have learned how to prepare, cook and store food in terra cotta clays like this, they withstand thermal shock better than vitrified stonewares and porcelains. Of course, more testing is needed, I will report as I proceed.

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).

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.

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).

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).

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.

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 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 an 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. The TSFL test on consistently size tiles can be used to log more precise results.

Refiring a terra cotta mug that had already bubbled only made it 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.

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).

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 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.

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.

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.

Problem at cone 1

These terra cotta clay bars were fired at cone 1. They have curled. That is ususual. It is happening because cone 1 is within the volatile firing range of this clay. Over a narrow range from cone 01-2 it shrinks drastically. These bars were fired on edge and the shelf heat-sunk the lower side, enabling the upper sections to get ahead (and shrink more, curling the bars). This is why this temperature would be avoided. Ware would warp. But at cone 02 it is stable. And at cone 2 it is also better. And vitreous.

Fine tuning the amount of Darvan in a terra cotta slurry

This is slurry is made of 80% Redart and 20% KT1-4 ball clay. It has very good casting properties IF it is deflocculated well (smaller pieces can be cast in 10 minutes and extracted from the mold in another 10). However high-iron slurries are notorious for gelling, either right away during mixing or later. Sometimes a slurry seems to pour well into a mold but 10 minutes later it has gelled so much that vigorous shaking is need to loosen it enough to pour it back out (even then much may hang on in corners). A batch of slurry may need extra deflocculant several times over the first few days of being used. Davan 811 is often used for these. More deflocculant is needed, in this case about 1 gram for each 100 grams of powdered clay.

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.

How do you decide what temperature to fire this clay at?

Test bars of a terra cotta clay fired from cone 06-8

It depends on the clay. This is a terra cotta body, L4170B. It is not like white-burning bodies, its "practical firing window" is much narrower than what looking at these fired bars and graph suggest. Do you need maximum density? On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But in-use pieces will warp and transparent glazes will be completely clouded with bubbles (from gases of decomposition generated by the body). What about cone 3? Its numbers put it in stoneware territory, water tight. But it still bubbles 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. It will even tolerate a silica addition to make it more durable. Then just add stains and opacifers to that. An engobe? Try matching L3685Z3 or L3685Z2 to it.

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.

Iron oxide as a debubbling agent at low temperature

One glaze is transparent, the other milky because it is filled with micro bubbles.

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. We did further tests and determined that a 1% addition also worked, but not as well. And screening out the larger particles slightly degraded the fining performance. So we have settled on 2% iron and screening the glazes to 100 mesh. Although iron works here, it will not always do so in other situations. And, other fining agent agents we have used at cone 6 do not work in this situation (e.g. 2% Zircopax, Alumina).

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

A terra cotta dish has been cut in half using a hand diamond saw

These saws are quite inexpensive and they make impressive claims on what you will be able to cut. But, things are different in practice! First, it is a real challenge to get the diamond wire to stay in the saw, it pulls out easily. It is not possible to turn the thumb screws tight enough with your fingers, you will need to use a pair of pliers. Then the challenge is tightening them enough to hold the wire securely, but not so tight you strip the threads. Once the wire is in securely, be prepared to make slow progress. This type of cutting is normally done by a machine that loops the wire continually, for hours. To cut this soft terra cotta piece took about 15 minutes! It was fired to about cone 010. Cutting stoneware or porcelain would likely be impossible. Even bisque ware fired to cone 06 or 04 would be harder than this. Using heavier gauge of wire would enable pressing harder, but this tool is not built heavy enough for that.

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.

Links

Glossary Low Temperature Glaze Recipes
In ceramics, glazes are loosely classified as low, medium and high temperature. Low temperature is in the cone 06-2 range (about 1800F-2000F).
Glossary Thermal shock
When sudden changes in temperature cause dimensional changes ceramics often fail because of their brittle nature. Yet some ceramics are highly resistant.
Glossary Clay Body Porosity
In ceramics, porosity is considered an indication of density, and therefore strength and durability. Porosity is measured by the weight increase when boiled in water.
Glossary Primitive Firing
The use of some traditional firing techniques is still popular among modern potters and sculptors (who are accustomed electric and gas kilns, often with computer controllers).
Glossary Majolica
Majolica is white opaque glazed red earthenware clay having colored overglaze decoration. But if you know more about what it is technically you will have more control of your product.
Glossary Bone China
A ceramic whose priorities are translucency, whiteness, fired strength and resistance to thermal shock failure.
Glossary Secondary Clay
Clays form by the weathering of rock deposits over long periods. Primary clays are found near the site of alteration. Secondary clays are transported by water and laid down in layers.
Glossary Earthenware
What is the difference between earthenware and a regular stoneware body? Earthenwares lack the glass development to fill voids and glue particles.
Media How to Apply a White Slip to Terra Cotta Ware
I will show you some secrets of making a base engobe (or slip) apply to leather hard terracotta ware in a thick, perfectly even layer.
URLs https://insight-live.com/insight/share.php?z=wK2ZW14yNo
Five low fire glazes: Which is the best?
Recipes L3724F - Cone 03 Terra Cotta Stoneware
An experimental Zero3 using Plainsman 3D clay
Articles Monoporosa or Single Fired Wall Tiles
A history, technical description of the process and body and glaze materials overview of the monoporosa single fire glazed wall tile process from the man who invented it.
Materials Iron Oxide Red

By Tony Hansen

Monthly Tech-Tip from Tony Hansen

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