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What Is Terracotta, Really?


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I was recently given a piece of a fired ceramic vessel by a friend who told me it was terracotta and asked me if I might be able to reproduce it.  The main reason this vessel was made of terracotta was supposed to be its relatively high porosity, which would allow water to evaporate through the walls of the vessel.  The piece of clay I was given felt a lot heavier than my garden flower pots so I decided to do a bit of research and run a few tests, in order to assess its "terracotta-ness".


After some digging it seems that terracotta is hard to pin down.  There appears to be a lot of qualitative discussion but  little in the way of technical benchmarks that one could measure against.  Any references or standards anyone knows of?  I am particularly interested in where terracotta ends and stoneware begins.  Many of the "terracotta" clay spec sheets I could find from vendors quote firing ranges (cone 5 and 6) which look much more like midfire stoneware than earthenware to me. 


Since I have always understood terracotta to be a kind of earthenware, I assumed it would melt or distort when fired to stoneware temperatures, but this did not happen.  Before firing a piece, I accidentally dropped it and it splintered/cleaved into two pieces which keyed  together perfectly over an area of about a square inch.  I fired both pieces sitting next to each other in a strong reduction firing (mains gas) to cone 10 well down, cone 11 half down.  Although the clay turned a noticeably darker chocolate brown color, it did not seem to distort or bloat much at all.  In fact those two pieces still keyed together afterwards in virtually the same way they did before the firing!  Essentially no change.


I could only find one general reference to the porosity of terracotta (Hansen), with a suggestion that is should be approximately 10%.  Does this sound right?  The spec sheets I was talking about above quote porosities around 5.5% to 8.5% level, but again that is firing to midfire temps.  The porosity of my shard seems to be between 5% and 6% when I tested it, which seems low to me, headed more in the direction of stoneware porosity.  However 5% or 6% seems to be just right for mid-fired "terracotta."  When I fired my piece to stoneware and again measured porosity, it had dropped to around 1%, which seems very close to the lower bound for a good tight stoneware body.


So is terracotta simply a stoneware body fired to a midfire range?   Or, or???  Thoughts?

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Terracotta is generally just an iron containing secondary clay fired at low fire temperatures.  They can be dug as is, or formulated from raw materials, but there's no definitive formulation.  A proper terracotta should be adequately mature at cone 06-04 and be fully vitreous around cone 1-2, not mid-fire temps.  Due to the high amounts of flux and iron, terracotta bodies are prone to bloat and warp when at maturation temperature.  It is possible, but undesirable for this reason, to mature a terracotta body to stoneware levels of porosity.  Very high loss.  If you've got a body that behaves well at cone 5-6, that's a mid-fire stoneware body, not something optimized for use as earthenware.


The best benchmarks are likely associated with performance and appareance, not composition.  A properly fired terracotta vessel will have a greater degree of shatter resistance than stoneware or even porcelain.  Low chip resistance, but excellent shatter resistance.  you could likely make a high-fire terracotta, but I don't know why you would.


But yeah, to my mind, it's a clay of a certain colour, with certain specific performance characteristics, and certain general production conditions.  

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Thanks Tyler, and most of what you have said jibes with what I thought terracotta to be, including the approximate firing range.  And I think you and I would agree that when you fire any earthenware clay - including terracotta - to stoneware temps it will not (should not?) be happy. 


However...since the first posting above I had a look at the spec sheets of some of the "terracotta" clays from well-know suppliers in the US,  and this has just added to my impression that terracotta is pretty much whatever you want it to be, can be fired to all sorts of temps with all sorts of porosities.  My personal favourite is an "earthen red" clay from an east coast vendor which they say can be fired at either cone 06 (for 13.4% porosity), or at cone 6 (!) for sub 1% porosity.   I guess if porosity is the key performance measure then this clay does it all. 


I seems likely that maturity of the body is the true test.   More specifically, it would seem that a mature terracotta body would have all the strength you are talking about, Tyler, but also display considerably more porosity than a mature stoneware body, ie, in excess of 10%.    Going to have to look more closely at what a mature terracotta body is like I guess and go from there.

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... and this has just added to my impression that terracotta is pretty much whatever you want it to be


And -- even more confusingly - sometimes the term is used based more on the nature of the object made

(rather than the body used or firing temperature).

In archaeology and art history, "terracotta" is often used to describe objects such as figurines not made on a potter's wheel.

Vessels and other objects that are or might be made on a wheel from the same material are called earthenware pottery; the

choice of term depends on the type of object rather than the material or firing technique.[6] Unglazed pieces, and those made

for building construction and industry, are also more likely to be referred to as terracotta, whereas tableware and other vessels

are called earthenware (though sometimes terracotta if unglazed), or by a more precise term such as faience.

... from https://en.wikipedia.org/wiki/Terracotta


Looking for clarity I consulted Architectural Ceramics, David Hamiltion.

The terms 'terracotta' and 'faience' are generally understood to mean, respectively, unglazed and

glazed earthenware. Contrary to the commonly held belief, 'terracotta' does not only refer to red-

firing unglazed clay but may be applied to any unglazed clay.


At least one factory creates a terminological dilemma by calling all hand-pressed facings, whether

glazed or unglazed, 'terracotta', and all cast or mechanically made pieces 'faience'.



Go figure?


PS The traditional Spanish ‘botijo’ seems to come in all sorts of colours. In my very limited experience 

they are made with quite thick walls, which I assume is a functional decision.


... this page includes an experiment keeping water cool in an oven, and a ref to an old technical paper.

After 7 hours in an oven at 39C the water inside the cooler was at 24C. Wow!


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This is a good example of how different industries/cultures have different definitions for the same term. For instance, the folks who paint little slip cast figurines call what they do 'ceramics', ignoring the fact that the word ceramics has a much larger definition. Commercial cone 6 glazes are referred to as 'high fire', whereas studio artists call cone 6 'mid range' and cone 10 'high fire'.


Commercial clays are often advertised as having a very broad temperature range, although they rarely work well at both ranges. Cone 6-10 stoneware clays are a good example- great at 10, underfired at 6. One could say any cone 6 clay is a good porous body at cone 04. In school we just used cone 10 stoneware clay for raku and it worked just fine, so maybe they should start advertising that.


Personally, I always defined terra cotta as a high iron, low firing body. Commercial clay and glaze companies are smart about using key words to define things in a way that will boost sales, hence cone 6 terra cotta, or cone 6 'shino' glazes. They kind of have a similar color to shino glazes, but that's about as far as it goes. Green shino? Really?!? Another is Amaco's Celadon line. Yes, they have a surface quality that is similar to celadon, and they are beautiful glazes, but none of them are what I would consider to be a true celadon. The problem with these labels is that they are falsely 'educating' some folks who don't know otherwise about what shino or celadon glazes really are.

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Guest JBaymore

And American Shino glaze is very different from actual Japanese Shino to start with.  The work of Virginia Wirt lead the western Shino exploration off in a direction that persists.... and is quite a bit different.  Then there is the whole "carbon trap" idea... that is NOT at all coming from the original tradition. 


Some nice glazes...... but calling them "Shino" ....... maybe misleading/confusing.





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To me, terracotta = red clay. More specifically, red clay garden pots.  I think it defines the colour more than the firing range/porosity or anything else.  If I go to a garden centre/nursery and ask for terracotta pots, they will be red.  Won't they?  Maybe it's also a language/country/culture definition.

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Yep, sure seems that terracotta is whatever you want it to be... 


OK, let me ask it another way.  Forget color and forget name.  If you wanted a clay that was strong but still had high porosity (say, around 15%) even when fired to maturity, what would you use?   

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I'm not sure how you're measuring porosity, e.g. voids in the fired clay or water absorption.

As I do a lot of earthenware I've tested a few clays, and the following may be of use. I tested unglazed bars firing up to the final temperature (which increased at 100 deg intervals) and soak for 15 mins.

Basically with all of them there was no significant shrinkage up to 1000. Then there was shrinkage for all between 1100 and 1200, and at 1300 the shrinkage of all was reducing as they were overfired.

If water absorption corresponds to your porosity, then with a 24 hour soak in cold water, at 1000 they all typically absorb 11 - 12% of dry weight, reducing to 6 or 7% at 1100, and 0.5 - 2% at 1200 and 1300.

So it loses its porosity quite a way before it has been fully fired to full strength - the opposite of what you want. However simple tests of how strong the fired clay is by how seeing how easy it is to break a thin fired test bar shows that the difference between 1100 and 1200 isn't huge.


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