Eleanora Eden on mon 28 aug 06
Thankyou Ron you answered my question exactly. I will happily not
see petalite in the pile of additives being contemplated.
In my crash course on understanding clay bodies I have so far come
to a very general picture that about 75% is various clays, half of what's
left is feldspar. You don't have a lot of room to play around with adding
stuff so what you do add has got to really perform!
Eleanora
>Hi Eleanor,
>
>First I should explain - why I would not use petalite - if I was adding
>Lithium to a glaze or clay - it does not have much Li2O - like about 4.3 -
>and it comes from South Africa I think - so it is expensive Lithium -
>spodumene would make more sense for us here in NA - as it would have more -
>over 7% Li2O and it is mined here. Even the Aussi spod would be a better
>deal - again - over 7% Li2O.
>
>If you are asking - what would happen if lithium (in the form of Petalite
>or Spodumene) were subbed into a body - at the expense of some of the spar?
>
>The expansion/contraction rate of lithium is much lower than the
>expansion/contraction rate of KNaO - so the expansion rate of that clay
>body would decrease.
>
>This is an advantage if the ware is to be subjected to uneven heating -
>lowering expansion will then lessen the chances of cracking due to heat
>shock. An advantage when making oven ware.
>
>Bodies with lower expansions need to have glazes of lower expansion if
>crazing is to be avoided - again - lithium can be used in glazes as well.
>
>If you are asking - will any clay body be made stringer with the addition
>of Petalite - that is different - I assume you mean - keeping the
>absorbency in the desired amount - and not just adding more flux and over
>melting.
>
>Will the body be stronger given the above caveat - I would not think so -
>but it would be more expensive for sure.
>
>RR
>
>
>
>>Hi Lili, Ron, and all,
>>
>>I have been asking these questions about clay additives and concluded that
>>the only use for petalite in a clay body would be for thermal shock resistance.
>>
>>It is not clear to me that a small addition of petalite (5%) would be
>>either good, bad,
>>or just indifferent in making up clay bodies for all around strength and
>>performance.
>>
>>Eleanora
>
>Ron Roy
>RR#4
>15084 Little Lake Road
>Brighton, Ontario
>Canada
>K0K 1H0
>
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www.eleanoraeden.com
Ron Roy on wed 30 aug 06
Hi Eleanora,
Yes - now that you mention it - each material you use should be though of
as having a specific function. I have not thought of it that way but I do
remember - Clay and Glazes for the Potter - Rhodes - lists the different
types of clays and why they should or should not be used in a body.
It is not as clear cut as he presents it - when you start testing ball
clays for instance - you will find a huge variation. Test data and fired
results including absorbency, shrinkage and colour all help when choosing
clays.
Knowing the clays your supplier stocks is a big help.
Most dealers stock OM#4 for instance - not a good clay to put in a body
simply because it varies too much - OK in small amounts but there are
better ball clays.
If you send me a list of clays available I will comment on which ones are
most stable and shrinkage and colour. You may even find your supplier can
provide some information sheets that will help you choose.
Probably best to also ask which materials they are thinking of not carrying.
I just did some quick calculation of some cone 6 and cone 8 throwing bodies
with 5% grog and they range from 86% to 77% clay (plastic material) so you
are in the ball park with you estimate. Remember - some clays - like Red
Art are not very plastic - more like a combination of plastic clay and
ground up shale - you may need to allow for that in some bodies. Adding a
more plastic clay will help in that case or maybe just adding more clay
period.
The best strategy is to use lots of different clays - so you can adjust
easily and no one clay can mess you up by varying too much.
And you didn't even ask a question - I think you know how to get me going.
Best regards - R
>Thankyou Ron you answered my question exactly. I will happily not
>see petalite in the pile of additives being contemplated.
>
>In my crash course on understanding clay bodies I have so far come
>to a very general picture that about 75% is various clays, half of what's
>left is feldspar. You don't have a lot of room to play around with adding
>stuff so what you do add has got to really perform!
>
>Eleanora
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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