Debbie on wed 25 may 05
Yes, freshly cast plaster CAN warp! When making large molds I leave them
supported and or clamped for about 24 hrs before separating and beginning
drying. This works for me. And importantly, they are always stored assembled
and clamped.
You might want to look into Hydrocal FGR-95 from US Gypsum. This can be used
with fiberglass matt for large molds. I've used this with great success for
large press molds. I use a splash coat of #1 pottery plaster against the
model then follow with several layers of the FGR-95 (fiberglass reinforced).
It is slow setting for the lay-up method of building. Molds are very strong,
thin & light weight. Google "Forton MG" (modified gypsum) to find out more.
I've purchased it from http://www.artstuf.com/home.html (but am sure you can
find it else where too.)
They say about it:
FGR-95: A very special product that is extremely versatile. Originally
intended for use as a laminating material with fiberglass for thin wall
construction, FGR-95 can also be cast (although the people at U.S. Gypsum
may not admit it) into a wide variety of mold materials. A bright white in
color, this material lends itself very well to accepting a multitude of
modifiers and additives. (see the section on Forton MG/Matrix G) Fine
aggregates can be used with it; metal powders can be used with it; the
possibilities are endless. It is even modified with a retarder to give you
ample working time, and when cured, has a ceramic like feel.
Best Wishes, Debbie
>
> My brain has been boggling as to how 2 halves of a mould could no
> longer fit
> together, having been cast against each other?! I haven't taken down the
> height of the joining edges at all, with the exception, of course, of the
> very outside edge, (i.e., not near the sculpture) which I've bevelled for
> ease of lifting, removed leakage from and generally tidied up in the way I
> usually would. Surely plaster can not warp, so how can this be?!
> Has any one
> else experienced this?
>
> Now to resolve it....I don't want to alter the joining edge anywhere near
> where the sculpture is pressed as this would ruin the finished
> form for ever
> more. Assuming there is some major fit problem elsewhere on the surface of
> the join, I could do with some way of seeing exactly where it's
> tight and so
> take it down a bit, and hopefully in doing so close the gap where it's
> 'baggy'. Would a contact paper like blue duplicating paper, (the sort of
> thing they have in invoice books), do the trick? Is there some other more
> obvious way to suss-out where the edges are too tight?
>
> Conundrum number two: Excited by working large scale again, (and given the
> above, possibly also slightly deranged), I have made another piece, a head
> 2'6" high, and this time, you'll all be pleased to hear, it is at least
> hollow. It's a flattish sort of head and easy in terms of seam
> lines, it is
> in fact a bigger version of the sculpture discussed above, just minus the
> body. Now it may be hollow but it is BIG, and if I were to cast it in the
> usual way, particularly because of the sloping back of the head,
> there would
> be a lot of surplus plaster in the negative spaces and it would weigh a
> #~@( of a lot. Now I may be strong in the arm, but my back has had a hard
> life, and my lovely assistant could do without a hernia. I've debated
> casting the plaster as we did at art school 20-odd years ago by
> pressing in
> shim to create the seam line all the way round, and lobbing plaster with
> gusto at the form ...obviously doing all this outside in the car park, as
> per the cat swinging problem. I'm just not convinced this would work with
> Herculite, (which goes off pretty quick), could be riddled with
> air pockets
> as it splats around, and even with strengthening webbing added in layers,
> may not have the structural strength to take being press moulded.
> The mould
> has got to be able to take being gently pummelled with a rubber mallet.
>
> Now I know that industry uses very large moulds for making such items as
> sanitary ware. I've seen pictures of moulds used to cast toilets etc, and
> they are not very thick. They're just about the sort of cross sectional
> thickness as I need to make mine, although from a different sort
> of plaster,
> I don't do slip casting. What I've never seen is how they make the moulds,
> or descriptions of how to either. Can anyone enlighten me, and so possibly
> prevent a repeat of conundrum number one? Failing that, can any
> one suggest
> a book/magazine/video/anything that actually discusses very large
> mould-making, as it's seeming that simply scaling up smaller mould making
> techniques are not working. I'd be very grateful.
>
> Baffled and weary, Ama in Totnes.
> Email: amamenec@lineone.net
> Website: www.amamenec.co.uk
>
>
>
>
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> __________________________________________________________________
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John Rodgers on thu 26 may 05
Large mold sections can warp. They should be well supported, and the
parts of the mold assembled together and clamped tightly at least for 24
hours, but actually best until dry.
Re-inforcing materials can be added to the plaster while still fluid,
that will increase strength. Hemp is one fiber material that is used and
is available through mold supply houses.
A good book about a big molding project is "The Black Canoe". It is a
story of the beginning of the world as viewed by the Indian peoples of
the northwest coast. Sorry, I don't have the authors name.
Regards,
John Rodgers
Chelsea, AL
Ama Menec wrote:
>Hi Clayarters,
>
>I have 2 plaster mould conundrums I'm hoping the wise and sensible can help
>me resolve. I am not unused to making plaster moulds, and have made a lot of
>them, but all the sculptures I have cast moulds for so far have been not
>much higher than 1' tall. But no problems with them, mostly 2-piece
>Herculite press moulds, made in the usual way and with plastic natches. Here
>is conundrum number one:
>
>I have recently cast a long thin sculpture, the figure being 3'8" tall, and
>nice and compact; a shape I have already cast before so I'm familiar with it
>'tho on a smaller scale, and cast as a two-part mould. I suppose the mould
>itself is just under 5' long. It has to be said this was possibly the most
>awful mould I've ever made, and in the most trying conditions I've ever had
>to cast in. Picture a space too small to swing a cat, rickety table to cast
>on, me, (being a 'traditionally built woman'), colliding with simply
>everything all the time, glamorous assistant, whilst delightful and sexy in
>her overalls, bloody hopeless at mixing and pouring plaster, and the second
>pour leaking down the side of the cottle. Oh, and not to mention the model
>was solid clay, (in the region of 50-75Kg), amount of plaster used simply
>enormous, and so at one point, with the blocking out clay, first half of the
>cast mould and the model, the total weight was in the region of 150 Kgs.
>Which promptly parted company with the board it was on as we turned the
>whole thing over to cast the other side, thanks to a thin coat of emulsion
>paint on the board I didn't realise was there, and of course the whole lot
>landed squarely on my knuckle. And no, I didn't swear; it hurt too much to
>utter a sound.
>
>OK, so it was all just horrible, but it sort of worked, although the outside
>of the feet end of both halves needed capping with more Herculite later on;
>one side because of the cottage cheese nature of the outside surface of the
>plaster thanks to lovely glamorous assistants' plaster mixing technique; the
>other because it was egg-shell thin due to lost volume of plaster due to
>leak, caused by crappy cottle building by me. The capping was not entirely a
>success, it's caused a few hairline cracks in the feet end but it will do
>for now. So I've chipped, hacked and surformed away as much of the overspill
>as I can stand, cleaned the mould thoroughly, and am ready to do the first
>press moulding.
>
>I decided to press mould just the head and neck area of the figure to start
>with, partly to try out the mould before committing to the total figure,
>partly because I'm still waiting for my Chinese Clayart wooden mallet to
>arrive for the hammering out of such long clay slabs, and partly because I'd
>like to use some heads as glaze tests for the full figure, and should any of
>them come out ok, they can be used as sculptures in their own right.
>
>So I press mould the head, and it comes out just fine; but I notice before
>we lift the top half of the mould off, that the moulds do not meet in the
>middle. The feet-end halves are tight to each other and so are the head-end
>ones, but from the shoulders to the ankles, (i.e. most of the figure), you
>can see daylight plainly through the gap, and it must be approaching 1cm in
>height. What I want to know is HOW? And of course, what to do about it.
>
>My brain has been boggling as to how 2 halves of a mould could no longer fit
>together, having been cast against each other?! I haven't taken down the
>height of the joining edges at all, with the exception, of course, of the
>very outside edge, (i.e., not near the sculpture) which I've bevelled for
>ease of lifting, removed leakage from and generally tidied up in the way I
>usually would. Surely plaster can not warp, so how can this be?! Has any one
>else experienced this?
>
>Now to resolve it....I don't want to alter the joining edge anywhere near
>where the sculpture is pressed as this would ruin the finished form for ever
>more. Assuming there is some major fit problem elsewhere on the surface of
>the join, I could do with some way of seeing exactly where it's tight and so
>take it down a bit, and hopefully in doing so close the gap where it's
>'baggy'. Would a contact paper like blue duplicating paper, (the sort of
>thing they have in invoice books), do the trick? Is there some other more
>obvious way to suss-out where the edges are too tight?
>
>Conundrum number two: Excited by working large scale again, (and given the
>above, possibly also slightly deranged), I have made another piece, a head
>2'6" high, and this time, you'll all be pleased to hear, it is at least
>hollow. It's a flattish sort of head and easy in terms of seam lines, it is
>in fact a bigger version of the sculpture discussed above, just minus the
>body. Now it may be hollow but it is BIG, and if I were to cast it in the
>usual way, particularly because of the sloping back of the head, there would
>be a lot of surplus plaster in the negative spaces and it would weigh a
>#~@( of a lot. Now I may be strong in the arm, but my back has had a hard
>life, and my lovely assistant could do without a hernia. I've debated
>casting the plaster as we did at art school 20-odd years ago by pressing in
>shim to create the seam line all the way round, and lobbing plaster with
>gusto at the form ...obviously doing all this outside in the car park, as
>per the cat swinging problem. I'm just not convinced this would work with
>Herculite, (which goes off pretty quick), could be riddled with air pockets
>as it splats around, and even with strengthening webbing added in layers,
>may not have the structural strength to take being press moulded. The mould
>has got to be able to take being gently pummelled with a rubber mallet.
>
>Now I know that industry uses very large moulds for making such items as
>sanitary ware. I've seen pictures of moulds used to cast toilets etc, and
>they are not very thick. They're just about the sort of cross sectional
>thickness as I need to make mine, although from a different sort of plaster,
>I don't do slip casting. What I've never seen is how they make the moulds,
>or descriptions of how to either. Can anyone enlighten me, and so possibly
>prevent a repeat of conundrum number one? Failing that, can any one suggest
>a book/magazine/video/anything that actually discusses very large
>mould-making, as it's seeming that simply scaling up smaller mould making
>techniques are not working. I'd be very grateful.
>
>Baffled and weary, Ama in Totnes.
>Email: amamenec@lineone.net
>Website: www.amamenec.co.uk
>
>
>
>
>--
>No virus found in this outgoing message.
>Checked by AVG Anti-Virus.
>Version: 7.0.322 / Virus Database: 266.11.17 - Release Date: 25/05/05
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>
>
>
>
Jeff Longtin on thu 26 may 05
Ama,
Warpage in big molds is always a problem. The best way to avoid it is too use
tepid, 85-90 degree?, water. This, more than anything, contributes to
warpage. (IMHO)
Next, are you mearsuring your water and plaster so you know they are
consistent? Measuring out large amounts of plaster and water is a pain, I know, but it
too can go a long way to minimizing warpage.
On eliminating the warp of your current mold: my only suggesttion would be
to soak the mold parts in water, so they're saturated, re-assemble them so
gravity can correct the warped piece, and let them dry. I've never done this
before, so I can't guarantee anything, but it's the only way I can see even
thinking of encouraging a correct rewarping.
Another question comes to mind: was there a long time before the first and
second sides were poured? SOMETIMES this can contribute to warpage as well.
take care
Jeff Longtin
Ama Menec on thu 26 may 05
Hi Clayarters,
I have 2 plaster mould conundrums I'm hoping the wise and sensible can help
me resolve. I am not unused to making plaster moulds, and have made a lot of
them, but all the sculptures I have cast moulds for so far have been not
much higher than 1' tall. But no problems with them, mostly 2-piece
Herculite press moulds, made in the usual way and with plastic natches. Here
is conundrum number one:
I have recently cast a long thin sculpture, the figure being 3'8" tall, and
nice and compact; a shape I have already cast before so I'm familiar with it
'tho on a smaller scale, and cast as a two-part mould. I suppose the mould
itself is just under 5' long. It has to be said this was possibly the most
awful mould I've ever made, and in the most trying conditions I've ever had
to cast in. Picture a space too small to swing a cat, rickety table to cast
on, me, (being a 'traditionally built woman'), colliding with simply
everything all the time, glamorous assistant, whilst delightful and sexy in
her overalls, bloody hopeless at mixing and pouring plaster, and the second
pour leaking down the side of the cottle. Oh, and not to mention the model
was solid clay, (in the region of 50-75Kg), amount of plaster used simply
enormous, and so at one point, with the blocking out clay, first half of the
cast mould and the model, the total weight was in the region of 150 Kgs.
Which promptly parted company with the board it was on as we turned the
whole thing over to cast the other side, thanks to a thin coat of emulsion
paint on the board I didn't realise was there, and of course the whole lot
landed squarely on my knuckle. And no, I didn't swear; it hurt too much to
utter a sound.
OK, so it was all just horrible, but it sort of worked, although the outside
of the feet end of both halves needed capping with more Herculite later on;
one side because of the cottage cheese nature of the outside surface of the
plaster thanks to lovely glamorous assistants' plaster mixing technique; the
other because it was egg-shell thin due to lost volume of plaster due to
leak, caused by crappy cottle building by me. The capping was not entirely a
success, it's caused a few hairline cracks in the feet end but it will do
for now. So I've chipped, hacked and surformed away as much of the overspill
as I can stand, cleaned the mould thoroughly, and am ready to do the first
press moulding.
I decided to press mould just the head and neck area of the figure to start
with, partly to try out the mould before committing to the total figure,
partly because I'm still waiting for my Chinese Clayart wooden mallet to
arrive for the hammering out of such long clay slabs, and partly because I'd
like to use some heads as glaze tests for the full figure, and should any of
them come out ok, they can be used as sculptures in their own right.
So I press mould the head, and it comes out just fine; but I notice before
we lift the top half of the mould off, that the moulds do not meet in the
middle. The feet-end halves are tight to each other and so are the head-end
ones, but from the shoulders to the ankles, (i.e. most of the figure), you
can see daylight plainly through the gap, and it must be approaching 1cm in
height. What I want to know is HOW? And of course, what to do about it.
My brain has been boggling as to how 2 halves of a mould could no longer fit
together, having been cast against each other?! I haven't taken down the
height of the joining edges at all, with the exception, of course, of the
very outside edge, (i.e., not near the sculpture) which I've bevelled for
ease of lifting, removed leakage from and generally tidied up in the way I
usually would. Surely plaster can not warp, so how can this be?! Has any one
else experienced this?
Now to resolve it....I don't want to alter the joining edge anywhere near
where the sculpture is pressed as this would ruin the finished form for ever
more. Assuming there is some major fit problem elsewhere on the surface of
the join, I could do with some way of seeing exactly where it's tight and so
take it down a bit, and hopefully in doing so close the gap where it's
'baggy'. Would a contact paper like blue duplicating paper, (the sort of
thing they have in invoice books), do the trick? Is there some other more
obvious way to suss-out where the edges are too tight?
Conundrum number two: Excited by working large scale again, (and given the
above, possibly also slightly deranged), I have made another piece, a head
2'6" high, and this time, you'll all be pleased to hear, it is at least
hollow. It's a flattish sort of head and easy in terms of seam lines, it is
in fact a bigger version of the sculpture discussed above, just minus the
body. Now it may be hollow but it is BIG, and if I were to cast it in the
usual way, particularly because of the sloping back of the head, there would
be a lot of surplus plaster in the negative spaces and it would weigh a
#~@( of a lot. Now I may be strong in the arm, but my back has had a hard
life, and my lovely assistant could do without a hernia. I've debated
casting the plaster as we did at art school 20-odd years ago by pressing in
shim to create the seam line all the way round, and lobbing plaster with
gusto at the form ...obviously doing all this outside in the car park, as
per the cat swinging problem. I'm just not convinced this would work with
Herculite, (which goes off pretty quick), could be riddled with air pockets
as it splats around, and even with strengthening webbing added in layers,
may not have the structural strength to take being press moulded. The mould
has got to be able to take being gently pummelled with a rubber mallet.
Now I know that industry uses very large moulds for making such items as
sanitary ware. I've seen pictures of moulds used to cast toilets etc, and
they are not very thick. They're just about the sort of cross sectional
thickness as I need to make mine, although from a different sort of plaster,
I don't do slip casting. What I've never seen is how they make the moulds,
or descriptions of how to either. Can anyone enlighten me, and so possibly
prevent a repeat of conundrum number one? Failing that, can any one suggest
a book/magazine/video/anything that actually discusses very large
mould-making, as it's seeming that simply scaling up smaller mould making
techniques are not working. I'd be very grateful.
Baffled and weary, Ama in Totnes.
Email: amamenec@lineone.net
Website: www.amamenec.co.uk
--
No virus found in this outgoing message.
Checked by AVG Anti-Virus.
Version: 7.0.322 / Virus Database: 266.11.17 - Release Date: 25/05/05
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