Lily Krakowski on sat 12 oct 02
With the greatest respect for Ron, I just want to add one note. All of us
clay elders remember when great clays and feldspars went off the market, and
yet, and yet clay body producers went on producing "Pimlico Red" or "Bluff
Buff". So although I am sure this happens less now that better chemical
analyses is practiced, one always should test each newly delivered clay both
with ones glazes, and for its firing properties,
Ron Roy writes:
> It is in the interest of education rather than an ongoing argument with any
> individual that I submit the following. When choosing clays be forewarned -
> especially in the face of conflicting information - test your clays to make
> sure they are suitable for the ware you want to make unless the make of
> those clays can show some accurate test data - even so I recommend building
> you own data. Remember - having a record of any particular clay body - over
> a period of time will tell you how well designed the body is and how
> resistant to variation over time.
>
>
>>Greetings all and Ron R.--- The Laguna catalog lists the absorption levels
>>for all of the stoneware clays at the cones they were designed to be used
>>at. They are listed at ^10 or ^5 firing levels, which ever one is that
>>specific claybodies' designated firing temperature. The absorption levels
>>for the stonewares listed vary from .5% up to 8%.
>
> In the Laguna catalogue WC398 (WS-4) is described as follows. Keep in mind
> - a customer complained this clay leaked when fired to cone 5.
>
> Page 11- Clay application Charts - WS4 clearly indicated for tableware and
> ovenware. It clearly states in the header that these recommendations are
> based on lab testing
>
> Page -16 - cone 5 stoneware clays - Similar to WS5 but with added flux for
> lower absorption - good to cone 8.
>
> (WS5 - care should be taken when used for functional ware - not totally
> vitrified in most cone 5 firings)
>
> RR comments - a range of 4 cones (45C or 113C) so it has to be either
> underfired to the point of leaking at 5 or overfired at cone 8 or both.
>
> Page 20 - approx. cone 5 Avr. water absorption plus or minus 2% = 1.5.
>
> RR comments - Jon says the 1.5 is a misprint and should be 5% - the burning
> questions are - what is the actual % absorption at cone 5 and 8 for this
> body.
>
> I'm not surprised that the person who sold this clay to Imzadi had no idea
> that this clay would not be suitable at cone 5 for the job - unless they
> had access to current test data. I can't think of any other reason than
> there is no current test data on this body.
>
>
>> Ron, I'm surprised you think it's more valuable to list that a body
>>reaches 3% absorption at ^8 3/4 than to state that it's absorption is 2% at
>>^10 or 5% at ^5. I would think that more potters are interested in the
>>absorption rate at the temperatures glazes are traditionally fired at than
>>at what specific temperature a clay reaches any given level of absorption.
>
> Why not say what the absorption rate is for every cone the body is
> recommended - or at least the upper and lower cones? That way anyone can
> choose the right clay they need.
>
>>It is absolutely wrong to say the companies I work for do not fill the
>>
>>needs of any potter - we give them what they want and if that is 5%
>>
>>absorption they get it. What we will not do is sell clay to potters making
>>
>>functional work that leaks.
>>
>> Well Ron, If I misread your earlier post I apologize, but I could have
>>sworn you stated you adjusted all the stonewares for the companies which you
>>consult to 3% absorption. But since those companies do have higher
>>absorption clays available, how in the world do you keep a potter from
>>buying one of your clays and using it any way that they decide to try and
>>use it???
>
> I never said that - I adjust each body to what I consider the right
> absorption rate for the cone it is to be fired at - usually under 2% - less
> for porcelains. Even our sculpture clays don't leak.
>
> Any one who wants clays with higher absorption get them as a special order
> - we don't sell them to anyone else - don't stock them. We also test all
> special orders and advise the customer if we fell it is necessary - for
> their application.
>
> If we did stock it we would clearly note the absorption rates in our
> catalogue and the test data would be available - any time - to any one - on
> any clay we make.
>
>
>>For definitions of clays see Rhodes Clay and Glazes - chapter 5 - all
>>
>>editions including Hoppers latest. First book I ever bought and an eye
>>
>>opener in the early days - still good basic information for those starting
>>
>>out.
>>
>> Correct me if I'm wrong again, but to me it seems what Rhodes is
>>defining in Chapter 5 of his book, 'Clay and Glazes for the Potter',(my
>>edition is the 13th from 1971) are raw materials which can be used to
>>develop clay bodies, not the bodies themselves. Chapter 5 is entitled
>>'Kinds of Clay' and describes Kaolins, Ballclays, Fireclays ect. and does
>>indeed include Stoneware clay and Earthenware clay, but as Raw or refined
>>Minerals that are dug from the ground and that can be used in clay body
>>composition. As an example Jordan clay is/was, a 'stoneware' type clay
>>mineral that comes to mind.
>
> The definition of stoneware is by temperature and over all shrinkage - a
> raw material can be classified as a stoneware - like a fire clay with
> enough plasticity to be thrown for instance. Jordan was not a stoneware by
> definition - for instance at cone 10 - to high shrinkage and too tight - it
> varied a lot so I can't say with certainty that it would not be at cone 6
> as well I'm sure not - it was a very sticky clay.
>>
>> In Chapter 6 Rhodes describes how to developed a body from those
>>minerals. He does give some sample stoneware bodies with absorption rates,
>>stating 5% as not being quite dense enough and 2% being too dense.
>> What Rhodes does not state is that Stoneware needs to be any specific
>>porosity rate, though he does give the above range. Which I would guess
>>leaves 4-3% absorption as "ideal". Though I don't believe it's "ideal" for
>>all purposes and Rhodes never actually states that it is.
>
> I believe 4% is too high - and 3% does not give enough leeway for variation
> of raw materials. I try to keep the stonewares between 1 and 2%. I learned
> the hard way but in the back of my mind was the fact that potters were
> relying on me to do it right - or they had to pay - and it's difficult
> enough to make it as a potter without having to try to do it with clay that
> is not right. I am very fortunate to have business men who agree with that
> and pay me to do it.
>>
>> In my opinion, the level of vitrification should not be the only gauge
>>used to determine at what temperature a clay body should be fired and I'm
>>not sure it would be the best way to categorize clays. Apparently there are
>>those who think by cone is a less than ideal method. I suppose there are too
>>many variables in ceramics and to use any single characteristic to
>>categorize clays is certainly an imperfect method.
>
> What would be a better way than to gauge by cone?
>
>> As I have stated, the levels of vitrification are listed in our catalog
>>for the approximate cones at which the clays were designed to function. So
>>are shrinkage, texture and color and a wealth of other pertinent info. The
>>more info you can digest before you start to work, the better off you are.
>>This type of info is indeed basic and should be available to those who are
>>going to use the clay. At least I can be in agreement with Ron on that
>>point.
>>
>>In closing-----You should always test your clays, glazes and minerals before
>>you commit to production, if they're not appropriate, try something else.
>>Life's too short to force the issue.
>>Jon Pacini
>>Clay Manager
>>Laguna Clay Co
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
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Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
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