Stephani Stephenson on wed 18 jul 01
Hi Craig
I think the clay is likely a contributor to your problem with the larger
slabs/tiles.
>From what you say it doesn't sound like drying or firing is the problem
Take a look at where the cracks are occurring. are they all in the same
place on each tile?
Sometimes a DIFFERENCE in thickness will cause a crack in a borderline
body. Some clay bodies( fine grained/ no grog) are not well suited for
the stresses and dynamics of large slab
pieces so wherever there is a weak point, say, a place where there is a
transition
from thick to thin, for example, these stresses will cause cracks.
so are the cracks in an area where the tile goes from thick to thin, OR
possibly a place where the tile is just too thin?
OR conversely is the tile too thick?
A smooth body clay will likely not be able to handle the variations OR
be able to
handle clay that is too thick. even 1/2 to 3/4 might be pushing the
limits.
I have not worked with Mesa. I know it is a nice color, but some of the
smoother low fire clays are less than ideal (to put it mildly) when it
comes to larger slabs/tiles
and/or work with variations in thickness or thick and large diameter
work..
you could decide to just re design and cut the tiles into thirds as you
say. Did you have any warping or was that OK?
other option is , if the cracks occurred in areas that were thick,
carve
in 1/8 to 1/4 inch from the back, sort of like a waffle pattern, so that
there is no place at all thicker than 3/8 or so, (or whatever thickness
seems to work.) leave 'bridges' on the backside that are thicker,
(difficult to describe) so
that the tile will not slump or warp. Not a very clear way to describe
it
but I hope you get my drift!
Otherwise adding grog sounds like a good idea.
For larger tiles, and for' forgiveness' with regard to thickness, you
need
a body with at LEAST 10 % grog, more like 15-30% depending on size. And
there ideally should be a mixture of 2 different grog sizes. Muddox grog
is OK for red bodies.
Aardvark also makes a clay called Dixon clay which is a nice terra cotta
color only is is SUPER Grogged and may be kind of over the top for you.
and it may be too porous at that temp.
On the other hand if you are pressing and not carving, the grog may be
pushed under the surface. It fires like a dream though, with regards to
warping and cracking
Have you asked Aaardvark if they have a grogged/sculpture version of
Mesa?
Usually if you order 1000 pounds, you can have it mixed on a custom
basis.
Of course you can work with these bodies but they are, as the saying
goes, somewhat 'unforgiving'. Or they have their limits and sounds like
you just found Mesa's limits. So you will either have to learn how to
adapt your work to those limits, (or stretch them as best you can), or
adapt the claybody so that it suit your work.
Anyway, you can baby a finicky clay along and maybe that is what you
will
have to do for this project.
though it seems like an enormous hassle it is part of the process when
embarking on a venture with new materials.
Whenever someone says, "Yes I think I will go into the tile business",
they almost always underestimate the learning curve and the time it
takes to adapt materials and techniques.
Try adding grog at least for one tile and see how it goes.
Meanwhile examine where the cracking is occurring to see if it has to do
with the
thickness/ thinness/transition from one to the other .
God luck!
Stephani
steph@alchemiestudio.com
Craig Fulladosa wrote:
> Hello,
> I hope I can bother you with another question. I attempted to make a
new
> tile mold (smaller). The first mold was about 11 x 16 and almost every
> tile had big cracks after the bisque firing. The thickness ranges from
> 1/2 inch to 3/4 of an inch. The new tile mold is about 8 x 12 inches
> with the same thickness. In the bisque firing all tiles fired fine I
> also fired all the border tiles which vary from 1/2 x 2 x 3 - 5
inches.
> None of those cracked in bisque or glaze. Now when I bisqued the large
> tiles all were fine but 2, six were without cracks. I glazed them all
> and almost every large tile cracked with 1/4 inch crackssometimes into
> three pieces. There was no crazing of the glaze. Do you think it is
> mainly a clay body problem. The clay has no grog. Should I add some
hand
> crushed grog of the same clay? 2 lbs to 25 lbs (10%) or what? Do I
need
> to fire slower, I already fired very slowly? Should I cut the tiles
into
> three sections and then mount together? Please let me know. I have
been
> potting a long time but haven't done alot of tiles. I have never had
> this problem before. This clay is new to me. A beautiful red lowfire
> clay - Mesa Red by Aardvark - no grog.
> Thanks
>
> --
> Craig
> The Clayman
> http://home.earthlink.net/~craigfull
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