Building A Large Thin Objects, Is It Better To Use Low-, Mid- Or Hig Temp. Clay?

[Mart]

Lets say you have to slipcast relatively large bowls. Top diameter (widest) is about 45 cm (18.3") and hight is about the same.

Final walls (after firing) can not be thicker than 6 mm and the shape is relatively complicated (not just a nice upside-down arch).

 

Q: Is it better to use low, mid or high fire clay?

 

Or lets ask this way: will low fire white clay warp less than a mid- or high temp white clay or half porcelain.

 

Pieces will be glazed with what ever the clay temp requires (forget the glaze for now)

[Norm Stuart]

As any clay body receives heat work, Wärmearbeit, toward its maturity cone, it densifies and shrinks. 

 

Fire any clay beyond its maturity cone and it will slump and warp.  If the temperature is raised enough it will melt.

 

Clay bodies with a lot of sand or grog, which melt at a far higher temperature than the clay are best for complex sculptures as the resulting clay has fewer voids to collapse collapse as the clay slumps and densifies.

 

Porcelains which contain a large percentage of larger particle sizes will take longer to deform, as the larger particles take more heat work to melt. It's analogous to clay bodies with sand or grog. Porcelains are often fired on a more heat resistant form, a "bowl setter" or plate setter", as porcelains become very plastic as they vitrify.

[bciskepottery]

Functional or non-functional? If functional, you will want to fire to vitrify the clay which could lead to some warping due to the thinness of the walls and the plasticity of the clay when fired to that temperature.

 

Low fire clays tend to warp less because they are not fired to maturity and don't reach that state of plastic-ness than clays fired to vitrified temperatures and which could lead to warping.

 

The thinness of the walls will be a challenge. Given the height involved, you need more thickness at the bottom to support the walls.

 

Is slip-casting with a paper-clay an option? More strength for handling at the greenware stage.

[Mart]

bciskepottery, strength at greenware stage is fine.

 

We already made the moulds, slip cast and fired few very good exemplar's but I started to think, maybe we can get an even better results with low/mid fire clay.

[Marcia Selsor]

I don't believe it has to do with low, mid or high fire clay as long as it is fired to the correct maturity.

Marcia

[Mart]

I don't believe it has to do with low, mid or high fire clay as long as it is fired to the correct maturity.

Marcia

Let me rephrase my question: What clay body gives us the most strength and least warping while underfired to achieve least warping/streching.

 

To make it more complicated, in next phase, we have to turn few bowls upside down and rest tiny part of the inside of the bottom at relatively narrow post. Reason for this is that we need to glaze the entire object so only a very small area inside is unglazed.

 

Huh... time for a small illustration:

 

[Marcia Selsor]

warping IMO would happen in over firing, not under firing unless it is very uneven.

I give your project is very form specific and specially designed for a particular function.

 

Marcia

[Mart]

warping IMO would happen in over firing, not under firing unless it is very uneven.

I give your project is very form specific and specially designed for a particular function.

 

Marcia

It happens even with slight under-firing. Try porcelain

[PeterH]

Mart,

You seem to be stretching the technology a bit with you combination of form, size, glaze coverage and "stilting". I'd be interested to know how do you stilt

the other bowls you mention, and how you display both sorts (with its implications for stilting). It sounds an interesting, if challenging, project.

.

For what it's worth I was reminded of something from a book, probably one of Tichane's. He was struggling to recreate some historic ceramic when he

suddenly had an insight into an old teacup he owned. It had a gilded rim, and he realised that this had not been introduced for artistic effect, but so that

the cup could be fired on its rim (to limit warping). The gilding was then added as a [low-fire] way of hiding/disguising the bare rim.

 

Could you find a way side-step your constraints?

 

Purely in brainstorming mode:

- What form does the warping take, and would thickening the base help?

- Would switching to high bisque, low[ish] glost help? You could support during bisque, and presumably warping would be reduced at the lower glaze temperature.

 

Regards, Peter

 

[Norm Stuart]

Mart -  I think you'll get a better handle on firing to the correct cone by buying some inexpensive Ortonkegels from a German supplier.

 

Firing to the "right temperature" is not the same as firing to the "right cone".

 

 

I don't believe it has to do with low, mid or high fire clay as long as it is fired to the correct maturity.
Marcia

Let me rephrase my question: What clay body gives us the most strength and least warping while underfired to achieve least warping/streching.

To make it more complicated, in next phase, we have to turn few bowls upside down and rest tiny part of the inside of the bottom at relatively narrow post. Reason for this is that we need to glaze the entire object so only a very small area inside is unglazed.

Huh... time for a small illustration:

firing_bowl.png

 

[Mart]

At the moment we get no noticeable warping in bisque firing and yes, we fire so the ware rest on the rim.

Those particular "bowls" can have unglazed rim so glaze firing was easy.

 

PeterH, we were thinking about doing something similar with our bowls too, but we need to get the glazes

match perfectly. This can be complicated.

 

Another path is firing the clay to almost maturity first and then glazing at mid temp.

Getting the glaze stuck to the ware when absorption rate is few % is not as east as it sounds. We must mess around with glaze "clue". But, I like that idea more than mixing 2 different temp glazes to perfect colour match.

 

I'll post some pictures of warping. It's a good example of what happens, when kiln shelves are not clean and smooth (not our kiln so we can not complain)

After that incident, we made huge slabs out of the same material and placed those under the bowls when bisque and glaze firing.

 

Norm, I'll get some Ortonkegels LOL, I like how this word sounds (obviously not using English pronunciation) because I do not speak any German)

[PeterH]

Mart,

 

I definitely was not suggesting trying to match two glazes with a seamless joint, as you suggest it sounds a rather thankless strategy. I was pointing out

that somebody had previously used a contrasting coating (in this case guiding) to disguise a discontinuity -- by turning it into a design feature.

 

If you are not heavily constrained by your choice of glaze, bisquing to maturity and then using a mid-fire glaze (with your existing stilting) certainly

sounds do-able. AFAIR the usual ways of applying glaze to non-absorbent bisque are: painting/spraying a glaze with added CMC or other gum;

or pre-heating the pot to encourage rapid evaporation (I assume that you paint or spray in this case as well). To me the former sounds the better

"production" strategy, as you are moving whole-heartedly from an absorption-based technology to a paint-film technology. [Given that your bowls

are large, and you probably want a very even coverage of glaze over a smooth surface.]

 

Regards, Peter

 

Totally tongue-in-cheek, have you thought of using cementation-style faience to get 100% glaze coverage?

http://www.rsc.org/ej/CS/2007/b606268g/b606268g-f12.gif

http://www.digitalegypt.ucl.ac.uk/faience/cementationmowie.html

http://escholarship.org/uc/item/9cs9x41z

http://www.yumpu.com/en/document/view/20352538/laboratory-production-of-egyptian-faiences-and-their-characterization

 

The first image is fig12 from

https://www.yumpu.com/en/document/view/6460429/the-invention-of-blue-and-purple-pigments-in-ancient-times

... it used to be downloadable from the RSC, but I can only find this online-readable copy at the moment