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Wide Range firing clay bodies


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The sharp dividing line in pottery is: functional vs. non-functional.

non- functional use is much more liberal and non- restrictive. There is much more wiggle in application. 

Functional work narrows down the freedoms into stricter parameters.  Thus the wide variance in responses. 


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18 hours ago, Polydeuces said:

Hey y'all,

Working with some cone 6 stoneware, interested in getting into the world of engobe. I've done quite a bit of research, but some things still seem relatively unclear. Here's a list of questions, I'd really appreciate some insight on the topic.

1. When is the best time to apply engobe? When the piece is leather hard? And, it needs to be thick?

leatherhard is the most common state for applying engobe.

2. What kind of surface is to be expected from common engobe recipes? I see a lot of slipware with glossy surfaces, but my assumption is that they were either A) fired in an atmospheric kiln or B ) coated in a clear glaze.

the easiest recipe you can use for an angobe is the clay body you are using. Slurry your trimmings, or just take some out of the bag and slurry it down to a creamy paste. Add  Mason " body stains" to it for color, or you can add oxide colorants ( copper, iron, or cobalt)  I know some who add china sand for texture. 

     2a) Because of the materials commonly present in engobe recipes, is it difficult to glaze over? Is it essentially an underglaze?

no different than glazing a standard clay body. Proper application is the real learning curve. Much of which is learning the proper consistency of the engobe.

3. Anything particular about obtaining particular colors? I was considering trying some mixes out using iron oxide and a few Mason stains.

try 3% iron and 5% black body stain. Body stains as shown below will not deviate or fade when fired. "body stains" are specifically noted on the Mason stain chart.


Thanks for your help~

gallery_73441_1082_1504522.jpgSputty: I have wondered for years why it was common practice in Europe to assign Cone 04 to 10 firing range on clay bodies: now I understand. Common practice becomes common acceptance.

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The reason we don't use cone 4-10 bodies here is because most cone 10 bodies will weep at cones 4-6. To say that your method has worked for you doesn't negate that fact. Search the forums here, we have people posting about weeping almost weekly. If it works for you, then great, keep at it. No one is telling you that it doesn't work for you. We're simply saying that it's not the best practice for people in general.

I take offense at the assertion that maybe 'we don't want the hassle of thinking, and want everything laid out for us just so'. Let's leave personal attacks out of this.

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I have recopied my reply to a posting of Neil's in the Engobe Quesions. I stated in my reply that I would be splitting the topic into two topics as the engobe discussion was hijacked by a discussion of strong feelings involving the use of wide range firing clay bodies. All of you in the US know that there exist several bodies that will fire from a range of ^4-@^9 or 10, no mater what the reason the manufacturer has for making it, potters need to be certain it fits their needs. . . . only testing will tell.

Years ago as and experienced, but naive potter, I used a 6-10 body from Standard Ceramics. This was a nice off white clay that took glazes well, and with spraying stains and glazes over a bristol glaze that matured at ^6 which I fired to a hard cone 6 seemed to work well. A few years went by, and I found that most of the pots I used personally started to weep, craze, and after some time glaze would lift from the piece, spalling.  Surfaces had become dull also. In the end, I had been involved in other things and when got back to ceramics, I rethought the entire clay firing range, glaze fit problem. Since then post made 15 years ago are still as they came out of the kiln after heavy use, microwave, oven and dishwasher use. They do not weep, even when not glaze on the bottom of bowls and other items.

The Engobe questions discussion had been subverted as will happen when opinions run strong in one direction or another. It seems that the engobes have been covered well, and presently there is little to add. Most that I see seems to be in the realm of personal agendas.Therefore, I have split the topic into two topics so as those wishing for information on Engobes may find it easily, and now there is a discussion on Wide Range firing clay bodies.  Those of us interested in functional ware may have a completely different viewpoint than those of you into sculpture, but we all need to understand preference and difference. 

If there are positive suggestions as to topics I have split or maybe missed, deal with me direct in a PM as I would prefer not to have further chance for animosity to occur within the forum. I would also be mindful of playing games with reporting members of the moderator staff as if one is a moderator. This game was played out a while back, and we lost an excellent resource, and there are still scars among the members of the forum, and some of us have very long memories.


best, once again


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To some degree I can speak for clay makers in the US: I know a few of them. I spent some time with a West coast clay maker at NCECA 2016 in KC. In particular we were discussing the evolution of Coleman porcelain: which by most measures is the industry standard on this side of the pond. So I had to ask the $64 question?  Why do some of the US clay makers still use a wide firing range in their specs? Answer: supply and demand- some demand 30 cents per pound clay, so we supply it.

yes Sputty, my nerdiness does not deal well with non specific applications or information: nor does it deal well with Wikipedia answers. I am interested in the responses from the clay makers you contacted. My question was sincere; I have genuinely been curious why I have seen so many posts stating a broad firing range?  There are many US makers engaged in the same practice. Yet I know from firing crystalline glazes that pottery does have its oddities. Run a crystalline glaze on a calculator, and the 35/1 SiAl ratio does not work- but yet it does. 

Slips are not on my research agenda yet, but I have used engobes for many years. There has been a long history of misapplication: slips being defined as engobes, or vice versa. Then again, there has never really been a defined distinction. Clay has 20-22% water content; increase it to 40% and you have a slip. So an engobe would land somewhere in the middle: which still places it above the plastic limits and into the liquid limits. Then again, everything I just stated could be dissected and dismissed by other definitions of use. Which is why 98% of potters just buy it all premade. In regards to clay, from my own research and testing: there is a defined molar percentage that produces a vitrified body above cone 3. Below cone 3, clay is just various degrees of tightly fused.


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 I am not going to get into a blame match here, but would appreciate a drop in the tone of rhetoric. Even though the outside world seems to have lost some of its civility, we need to hold on to it here to make this a place for answers, and information. Often the answers come in the way of opinion that one must make their own decision as to whether to credit it or not. However, in the interest of encouraging others to post, best not to discredit, just because something is different from one country to another.

So for the interest of information you may find the following link of interest. Standard Ceramics is one of the larger suppliers of clay bodies and materials in the US. 


You may find it interesting to note that the largest firing range clay bodies is in the Raku section. 

I would also note, that there is a great deal of decision making on the part of the potter in choosing a clay body for their work as the extensive listing always amazes me every time I decide to try something new. SC has always been supportive in that decision making as when asked will supply small samples for testing, which I do. I am certain I am not alone there.




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Most of the clays with wide ranges of temps are going to weep at many of those temps.At least thats what I have found to be true.

I'm sure there is an optimum temp and below that its maybe working but not well and above that the same may be true and at some point bloating/slumping will come into play.

I have only been around of two such bodies  in past 45 years and that stated range was really not true unless you made non-functional wares from it. Speaking of non -functional wares its does not matter much at all as long as it holds together . 

One of the hardest parts is finding glazes that work and will not weep in a clay that is not mature.

For functional wares it easier to use a clay at the temp you want to fire to and where the glazes fits well and the product do not weep or the glazes do not cause problems.

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Are you are saying 1% absorption rate  at cone 4-10 thought that whole range. And the maturing temp is the whole range as well.Meaning its just as strong throughout that range?Just really hard to think the EU has something that the rest of world has not come across in ceramics?

Asia really has us all beat by centuries and the EU sneaked ahead in just the past 15-20 years?

Next world cup will hosted by Eskimos in Alaska I hear in 2030.

I'm having hard time wrapping around this really all all joking aside

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Just now, Sputty said:


I am told that the clay has been developed to have less than 1% absorption at 1180°C. It can then be fired between the lower temp to the maximum before the clay would start to bloat, which in this case is some 1300°C. 


This explanation from EU suppliers actually makes sense. I know kaolin and ball clays for the most part have a cone 32 rating without fluxes. So formulating to vitrify at roughly a high 5, low six makes sense: as I also know it will easily handle several cone above that. Thin wall,  large angular pieces might have issues: but the majority of work will tolerate it. I formulate and use a cone six, but have fired it to cone nine: so I get it.

I think I need to clarify however: when I originally posted about broad firing ranges: that was in reference to cone 04 to cone 10. Formulating  a cone six and firing to cone 10 I get: the rest I do not. On the other spectrum: formulating for cone 10-12 and expecting vitrification at cone six: sets my alarm off as well. Thank you for inquiring from your local sources: a question I have long pondered.

i think it should also be noted that kaolin and ball clay from Europe tend to have higher spar levels, and often finer grains. Hematitie is also the source of iron in natural clays: so some variance in formulation standards should be expected. All of which would play some minor roles in vitrification temps.


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