Jeff Lawrence on sat 17 may 97
Harvey Sadow asks:
> Have you tried making an open backed one piece slap mould? Fill it to
>the top and let it set up. Turn it over and let it drop out. Why do
>you need to pour a two piece mould for a slab at all, Jeff?
>
>Harvey
>
Good thinking, in perfect synch with my phase one.
This approach had two fatal flaws I recall (might have found more but then
the mold got kicked over and instigated phase two, which you read about).
Those flaws:
(1) open side is ugly, with swirls and irregularities that get worse as it
dries.
(2) slab dried unevenly, with the edges swooping down to the middle in a
caricature of a miniscus from the very start. As they dried, they reached
for the sky while the sodden middle hunkered down.
by comparison, the two-piece product is as smooth as the original -- should
have used something with two smooth faces! -- and uniformly dry/hard throughout.
Jeff
Jeff Lawrence
Sun Dagger Design
ph/fax 505-753-5913
Suzanne Wolfe on sun 18 may 97
There has been some discussion about casting slabs. Two types of molds
have been suggested:
> Harvey Sadow asks:
> > Have you tried making an open backed one piece slap mould? Fill it to
> >the top and let it set up. Turn it over and let it drop out. Why do
> >you need to pour a two piece mould for a slab at all, Jeff?
Jeff replied that this technique has two flaws:
> (1) open side is ugly, with swirls and irregularities that get worse as
it dries.
This is true to some extent. Some of the swirls and irregularities can be
prevented by how you pour the mold. The best way is to elevate it
slightly from one corner, and then pour directly at that corner. To
prevent a hard spot from forming, you need to pour onto a sheet of paper,
which is removed after most of the slab is poured. The elevated end is
then lowered so that the whole slab is level.
Swirls tend to develop more if you pour the slip "all over the place",
rather than just from one place, from where is moves out evenly across the
surface of the mold.
> (2) slab dried unevenly, with the edges swooping down to the middle in a
> caricature of a miniscus from the very start. As they dried, they reached
> for the sky while the sodden middle hunkered down.
This is true also, although I solve that problem by cutting away the
slightly thicker edges after the slab is dropped out of the mold. (I use
a piece of drywall against the mold, and just flip it.)
> by comparison, the two-piece product is as smooth as the original --
should have used something with two smooth faces! -- and uniformly dry/hard thro
Absolutely true. However, I have two problems here. Although the faces
are really smooth, when the slabs are cut, there is a definite line right
through the middle of it, which indicates the manner in which the clay
particles have aligned themselves in the mold -- one side to the top half
and the other to the bottom half. This line down the middle is a definite
weak spot, and if you tweak the slab to break it, it often breaks unevenly
along that middle face. This is not so bad, except as you cast the molds
more and more -- that plane through the middle becomes more and more
pronounced. I have had slabs that have empty spaces right in the middle
(even though I have a big spare, which I always keep filled up). So,
while it is true that the two-sided mold does produce the most "beautiful"
slabs, they are not without drawbacks. Frankly, I think rampressing would
be best.
Suzanne Wolfe
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