animal bones shards as stilts for stoneware?

[Emilio]

Is it possible to use animal bones chopped shards as kiln stilts for HT wood firings, instead of shells?

[Min]

Hi and welcome to the forum.

Bones won’t survive the heat of the kiln. (think cremation)

edit: I was rethinking what I wrote and looked up using bones for stilts. Came across the following article, turns out you can you bones. Link is behind a paywall but you can access 3 free articles a month.

https://ceramicartsnetwork.org/daily/article/Wadding-for-Wood-Firing

Edited December 26, 2023 by Min

[PeterH]

I've no practical knowledge, so treat this all a here-say

I'm interested in how you get on, but ...

- I don't think either bones -- or seashells -- will survive high firing sufficiently intact to act as a stilt.
- I doubt that bones will help form a "glaze" in the same way that seashells do.

Firstly I think shells are used as a separator between the pot and clay wadding placed inside the shell. Shells seem a much better shape for this than most bones.
Effective Side Firing
https://ceramicartsnetwork.org/pottery-making-illustrated/pottery-making-illustrated-article/Effective-Side-Firing#
While wadding is traditionally only used in atmospheric kilns to prevent pots from sticking to the shelves, it is necessary for side firing in electric kilns.  Sea shells turn to powder (calcium oxide) during the firing, and will not support the weight of a pot.  Using wads under the shells prevents the pot from falling onto the drip tray and sticking to it.

Secondly the chemistry is different, perhaps critically so.
Shells in Ceramics
https://www.weloveclay.com/read/53007/53007/
Why then were shells not adopted more widely? Australian wood fire potter Owen Rye has pointed out that though ancient potters were keen observers of their craft, none could have understood or intuited the science of shell use. The shells themselves are formed from calcium carbonate which, when heated, converts to calcium oxide or ‘quick lime’, a powerful flux. In isolation, calcium is one of the most refractory oxides, only melting at 2572˚C+. But combine that calcium with alumina and silica and you have what is known as a ‘eutectic’ – a combination of oxides that melt at a lower temperature than they would on their own (in this case, a much more manageable 1170˚C).

Why then were shells not adopted more widely? Australian wood fire potter Owen Rye has pointed out that though ancient potters were keen observers of their craft, none could have understood or intuited the science of shell use. The shells themselves are formed from calcium carbonate which, when heated, converts to calcium oxide or ‘quick lime’, a powerful flux. In isolation, calcium is one of the most refractory oxides, only melting at 2572˚C+. But combine that calcium with alumina and silica and you have what is known as a ‘eutectic’ – a combination of oxides that melt at a lower temperature than they would on their own (in this case, a much more manageable 1170˚C).
All this means that on the interface between shell and pot there is a melt that leaves a shell-like scar on the surface of the clay. As the shells are stuffed with clay to prevent them from collapsing, the same happens on the inside. The core of the shell remains pure calcium, which is dry, friable, expands when wet and can be easily removed after the firing. An aesthetic bonus of using seashells is that they contain small quantities of salt. During the firing, this salt can volatilise, leaving a subtle halo of salmon pink or orange on lighter coloured clay bodies.

 

Shells decompose into calcium oxide, bone eventually decomposes to a mixture of calcium oxide and phosphorous pentoxide, which may well prevent a suitable eutectic forming. Also bone ash tends to retain some of its structure, probably making both oxides less available for forming a "glaze".
https://en.wikipedia.org/wiki/Bone_ash
Bone ash is a white material produced by the calcination of bones. Typical bone ash consists of about 55.82% calcium oxide, 42.39% phosphorus pentoxide, and 1.79% water.^{[clarification needed]} The exact composition of these compounds varies depending upon the type of bones being used, but generally the formula for bone ash is Ca₅(OH)(PO₄)₃. Bone ash usually has a density around 3.10 g/mL and a melting point of 1670 °C (3038 °F). Most bones retain their cellular structure through calcination.

 

 

Edited December 26, 2023 by PeterH

[fergusonjeff]

Peter,

  I may be reading it wrong, but I think the original poster was referring to using bone and shell as wadding rather than as posts.  Shells work great for wadding during the firing, but within a few days turn to powder.  I imagine bone would be similar, but maybe less structurally stable during the firing.

Jeff

[Min]

19 minutes ago, fergusonjeff said:

Peter,

  I may be reading it wrong, but I think the original poster was referring to using bone and shell as wadding rather than as posts.  Shells work great for wadding during the firing, but within a few days turn to powder.  I imagine bone would be similar, but maybe less structurally stable during the firing.

Jeff

That’s the way I read it too. In the link I posted Simon Levin wrote of a fellow woodfirer who used thin slices of cow bones wadding.

[Callie Beller Diesel]

I do have practical knowledge of this. It’s wise to back up shells with other wadding for structural support, especially when tumble stacking, placing large ware,  or other instances where the load may shift. The remaining calcium is pretty hard immediately after firing, but if the shell itself was thin, there’s only so much structural load it can take. So size and shape of the proposed bone is something to consider. The Simon Levin article makes mention of his neighbour using cow bone sliced on a bandsaw, but something like poultry bones might not work as well on their own, being smaller and much less dense. 

According to the digitalfire bone ash page, the frit softening point, which will be affected by the fact that it’s pulverized, is 1670*C/3038*F. So unless the bone comes into contact with ash or other fluxes, I can’t see it melting or fusing to things it isn’t supposed to.

You could get some interesting flashing from using bone that other wadding options might not offer. Bone ash is used in some high fire iron reds, which is another interesting connection.