Tom Wirt on sat 28 nov 98
-----Original Message-----
From: Ron Roy
----------------------------Original message----------------------------
Testing glazes for fit on a body it not that hard to do - but you need to
have some assurance that the clay will be at least similar from batch to
batch - I do recommend testing every batch of clay you buy - if there are
no batch numbers on the box - don't buy it. In fact knowing what I know now
- if the manufacturer is not testing their product it's a gamble and you
will have to pay sooner or later - unless you do the testing before you use
it.
RR
Question:
Ron....What tests do you recommend for the individual potter who has no
equipment more exotic than a ruler (for shrinkage) posts to test slumping of an
extruded rod, heating and freezing (to test glaze fit)?
Also, how do we take into effect variations within a batch. My supplier stamps
a make date on the box, but variations in blending time dry and wet), or when
they go into a new batch of one of the raw materials during a run, even bag
weight variation of the raw materials give great (sometimes visible and
certainly tactile), differences within a batch date.
We're having a problem with getting a large flat bottom (12+") baking dish to
work. It will pass the freezing/boiling water test repeatedly, but if put at
room temperature into a 450 degF. oven, will crack across the bottom. I do all
the recommend procedures in making. (even thickness, bottom compression, slow
drying, curved intersections, etc.) The body is a standard fireclay with kyanite
added for thermal shock improvement. (As near as I can tell the blend is AP
Green fireclay, OM4 ball clay, Custer spar, EPK, and kyanite). As usual, the
actual formula is not released.
There is some slight crazing of some of our glazes which we haven't cured yet,
but if I read your note, this isn't what's probably causing the crack. Or is
the oven test to severe without something in the pot?
Any help will be much appreciated. My goal, as I've mentioned to you before is
to develop my own blend so at least we know what's in it. But I haven't gotten
there yet. Then we'll work with you for full testing and reformulation of
glazes for fit.
Tom
Ron Roy on wed 2 dec 98
Hi Tom,
The testing we do is really simple and there are more complicated tests but
- they have served us well over the last 20 years.
Make some bars of any new clay - at least two. 6" long, 1/2 thick and 1"
wide. Make a line 10 CM long. Actually we use a line 5" long using an inch
scale divided into 10ths (get at a drafting supply place) - you have a
longer line which means a bit more accuracy.
let dry and bisque fire - measure line - if it's 9.5 then you have a wet to
dry shrinkage of 5%. Then fire in a normal firing to glaze temp - measure
again - if the line is now 8.8 you have an overall shrinkage of 12% - just
about right for throwing. I recommend having a cone near the bars - mark
the cone bend on each bar as it comes out of the glaze fire with something
that won't boil off in water. Weigh the bars as they come out of the kiln -
right away so they don't have any chance to pick up any moisture form the
air (or wrap em up in plastic) mark the weight on the bar leaving room
above to put in the wet weight.
Boil in water for 2 hours keeping the water level over all the bars. Cool
with water till all are cold - pat dry - weigh again - mark wet weight on
bar above the dry weight. Subtract dry from wet.
Dry weight divided into difference times 100 gives % absorption.
Wet weight - 153.8
Dry weight - 150.5
Diff - 3.3
3.3 divided by 150.5 x 100 = 2.2 - a little over but I would probably not
monkey with this unless it seems to be a trend. Average the two samples -
if one is quite different then maybe there was air in the clay - better to
do a number of bars - the averages will give a better picture.
I try to keep the stone wares between 1 and 2% - porcelains near 0.
Re your supplier and continuous batches - I would talk to them and ask them
if they would be willing to pull some consecutive boxes for you. Another
solution would be to blend whatever you get and then test - you would need
a decent pug mill of course. You could take random samples to test but what
to do if they weren't similar.
The cracking baking dishes - sounds like cristobalite to me - you can find
out quick enough with a dilatometer - I am making the assumption you are
firing to cone 10 - if its cone 6 there is rarely enough cristobalite to be
a problem. Ask if they are adding any silica to the body. Glazes which
craze are not know for cracking pots so I have to assume it's something to
do with the body.
At the risk of putting most of you into a deep sleep - just a bit more.
Cristobalite goes through it's reversible inversion at oven temperatures.
So some parts of the dish get heated faster - the cristobalite gets bigger
in that part faster than the rest. If you do use hand made casseroles - put
them in the oven as dead center as possible to try and even out the
heating.
Be glad to help you design a body Tom - having the dilatometer has made an
enormous difference in understanding what goes on. Maybe your clay company
would mix it for you - maybe they should be talking to me - or at least
have their clays measured.
R
>Ron....What tests do you recommend for the individual potter who has no
>equipment more exotic than a ruler (for shrinkage) posts to test slumping of an
>extruded rod, heating and freezing (to test glaze fit)?
>
>Also, how do we take into effect variations within a batch. My supplier stamps
>a make date on the box, but variations in blending time dry and wet), or when
>they go into a new batch of one of the raw materials during a run, even bag
>weight variation of the raw materials give great (sometimes visible and
>certainly tactile), differences within a batch date.
>
>We're having a problem with getting a large flat bottom (12+") baking dish to
>work. It will pass the freezing/boiling water test repeatedly, but if put at
>room temperature into a 450 degF. oven, will crack across the bottom. I do all
>the recommend procedures in making. (even thickness, bottom compression, slow
>drying, curved intersections, etc.) The body is a standard fireclay with
>kyanite
>added for thermal shock improvement. (As near as I can tell the blend is AP
>Green fireclay, OM4 ball clay, Custer spar, EPK, and kyanite). As usual, the
>actual formula is not released.
>
>There is some slight crazing of some of our glazes which we haven't cured yet,
>but if I read your note, this isn't what's probably causing the crack. Or is
>the oven test to severe without something in the pot?
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Marvin S. Flowerman on sun 6 dec 98
Hi All:
I want to try a recipe given to me by a fellow potter calling for Frit 3417.
My glaze reference indicate that this is a lead based frit.
Can you recommend a substitute, not lead based?
The other ingredients are potash feldspar, dolomite, fluorspar, kaolin, tine
oxide and vanadium oxide.
Any help would be appreciated.
Marvin Flowerman (marvpots@aol.com)
Ron Roy on tue 8 dec 98
If the lead is replaced with say boron I am sure it will not be the same -
but have no experience in this area.
Watch out for that Vanadium - not frienly at all - my advice is to pass on
this one.
RR
>----------------------------Original message----------------------------
>Hi All:
>
>I want to try a recipe given to me by a fellow potter calling for Frit 3417.
>My glaze reference indicate that this is a lead based frit.
>
>Can you recommend a substitute, not lead based?
>
>The other ingredients are potash feldspar, dolomite, fluorspar, kaolin, tine
>oxide and vanadium oxide.
>
>Any help would be appreciated.
>
>Marvin Flowerman (marvpots@aol.com)
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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