John Rodgers on fri 21 jan 11
May,
Depending on the shape and size of the mold, you should be able to do
that. Best not to let work completed dry out, however. Plaster bonds to
plaster very well if the contact surfaces are clean, moist and free of
debris. It would not hurt to roughen up the contacting surfaces to give
the new plaster a place to get a firm grip.
Good luck,
John
John Rodgers
Clayartist and Moldmaker
88'GL VW Bus Driver
Chelsea, AL
Http://www.moldhaus.com
On 1/21/2011 6:45 PM, May Luk wrote:
> Hello all;
>
> I have a bit of a plaster disaster today. Ran out of potter's plaster
> on a tall form (had a bad leak on a weak wall due to gravity) I have a
> bag plaster for tile press. Can I use this to finish my mold? 3/4 of
> it is cover and and the model is well coated with potter's plaster.
>
> If I cannot mix the plaster types, can I just leave the unfinish
> project and wait for my plaster order to arrive next week and finish
> the mold?
>
> Many thanks in advance for your answers.
>
> May
>
> --
> http://twitter.com/MayLuk
> http://www.takemehomeware.com/
>
>
May Luk on fri 21 jan 11
Hello all;
I have a bit of a plaster disaster today. Ran out of potter's plaster
on a tall form (had a bad leak on a weak wall due to gravity) I have a
bag plaster for tile press. Can I use this to finish my mold? 3/4 of
it is cover and and the model is well coated with potter's plaster.
If I cannot mix the plaster types, can I just leave the unfinish
project and wait for my plaster order to arrive next week and finish
the mold?
Many thanks in advance for your answers.
May
--
http://twitter.com/MayLuk
http://www.takemehomeware.com/
John Rodgers on sat 22 jan 11
May,
An additional note - This is generally not a good idea if the mold is
to be used for slip casting. For that - the mold must be of all the same
plaster mix to ensure equal pull of the water from the clay. For other
types of clay work, it should work fine.
JOhn
John Rodgers
Clayartist and Moldmaker
88'GL VW Bus Driver
Chelsea, AL
Http://www.moldhaus.com
On 1/21/2011 6:45 PM, May Luk wrote:
> Hello all;
>
> I have a bit of a plaster disaster today. Ran out of potter's plaster
> on a tall form (had a bad leak on a weak wall due to gravity) I have a
> bag plaster for tile press. Can I use this to finish my mold? 3/4 of
> it is cover and and the model is well coated with potter's plaster.
>
> If I cannot mix the plaster types, can I just leave the unfinish
> project and wait for my plaster order to arrive next week and finish
> the mold?
>
> Many thanks in advance for your answers.
>
> May
>
> --
> http://twitter.com/MayLuk
> http://www.takemehomeware.com/
>
>
Snail Scott on sat 22 jan 11
On Jan 21, 2011, at 6:45 PM, May Luk wrote:
> ...Ran out of potter's plaster
> on a tall form... I have a
> bag plaster for tile press. Can I use this to finish my mold? 3/4 of
> it is cover and and the model is well coated with potter's plaster...
What kind is the other plaster? Regular plaster of Paris?
What is this mold for: press-molding or slip-casting?
For press molding, darn near anything will do, but for
slip casting, the lesser absorption of all other types of
plaster will result in a thinner clay wall in those regions
of your your cast object. Plaster of Paris isn't hugely
different, but Hydrocal, for instance, would be quite bad.
Another issue is the discontinuity between the new and
old layers of plaster, even if they are the same kind.
Even if well bonded together (which isn't guaranteed, of
course), plaster for slip-casting relies on capillary action
to wick the moisture out of the slip and carry it through
the thickness of the mold. The moisture may not travel
so well across the joint as through a solid continuous
plaster thickness. Probably not as big a factor as the
plaster type, but still a potential problem, possibly
contributing to thinner castings in that area of the mold.
If you do elect to continue and use the mis-matched
plaster, try this to improve the bond between them:
1. Score the surface where you intend to add more
plaster. Score it deeply and roughly, but widely, so
that the new plaster will fill the marks and act like
finger joints or dovetail joints in wood. Make no
skinny gouges like knife marks; wide ones like
from a small loop tool are better.
Wet the old plaster very well before applying the
new plaster, otherwise the old plaster may suck the
moisture out of the new plaster before it's fully set,
and it needs all its moisture for its chemical hardening
process to work properly.
Adding the new plaster as soon as possible improves
the chances of a good bond, but moistening the plaster
will minimize the importance of that aspect.
If I wanted maximum slip-casting quality, I would try to
remove the current plaster, and start over from scratch
when the new shipment arrives. If that would damage
the pattern, then leave the current plaster in place, keep
it wrapped up and damp, and add to it when the new
stuff arrives. Using a mismatched plaster would be my
least favorite choice, unless you are only using it for
press-molds. For that, it would likely work fine.
-Snail
May Luk on sat 22 jan 11
Thanks for all your input.
I went in the studio and chucked everything. Yesterday was a long day.
I laminated a 4'x8' table. Had a plaster disaster (5 ltr water, 6500
gm plaster and few lbs of clay wall) 40 deg cold studio with no hot
water.I woke up with sore muscle (didn't sleep well at all because of
this) and a bad headache. I decided this is not how I can work. I
can't slip cast this biggish form even the mold was made well. I don't
have the right set up and no assistants. I decide I will only slip
cast small objects and handbuild bigger forms.
This is the lesson I learned this weekend.
Also I noticed I always get a bad headache sore arms and a slight sore
throat after physically demanding days at the studio. I never get that
when I'm on vacation!
Thanks & Best Regards
May
On Sat, Jan 22, 2011 at 3:15 PM, Snail Scott wr=
=3D
ote:
> On Jan 21, 2011, at 6:45 PM, May Luk wrote:
>> ...Ran out of potter's plaster
>> on a tall form... I have a
>> bag plaster for tile press. Can I use this to finish my mold? 3/4 of
>> it is cover and and the model is well coated with potter's plaster...
>
>
> What kind is the other plaster? Regular plaster of Paris?
> What is this mold for: press-molding or slip-casting?
> For press molding, darn near anything will do, but for
> slip casting, the lesser absorption of all other types of
> plaster will result in a thinner clay wall in those regions
> of your your cast object. Plaster of Paris isn't hugely
> different, but Hydrocal, for instance, would be quite bad.
>
> Another issue is the discontinuity between the new and
> old layers of plaster, even if they are the same kind.
> Even if well bonded together (which isn't guaranteed, of
> course), plaster for slip-casting relies on capillary action
> to wick the moisture out of the slip and carry it through
> the thickness of the mold. The moisture may not travel
> so well across the joint as through a solid continuous
> plaster =3DA0thickness. Probably not as big a factor as the
> plaster type, but still a potential problem, possibly
> contributing to thinner castings in that area of the mold.
>
> If you do elect to continue and use the mis-matched
> plaster, try this to improve the bond between them:
>
> 1. Score the surface where you intend to add more
> plaster. Score it deeply and roughly, but widely, so
> that the new plaster will fill the marks and act like
> finger joints or dovetail joints in wood. Make no
> skinny gouges like knife marks; wide ones like
> from a small loop tool are better.
>
> Wet the old plaster very well before applying the
> new plaster, otherwise the old plaster may suck the
> moisture out of the new plaster before it's fully set,
> and it needs all its moisture for its chemical hardening
> process to work properly.
>
> Adding the new plaster as soon as possible improves
> the chances of a good bond, but moistening the plaster
> will minimize the importance of that aspect.
>
> If I wanted maximum slip-casting quality, I would try to
> remove the current plaster, and start over from scratch
> when the new shipment arrives. If that would damage
> the pattern, then leave the current plaster in place, keep
> it wrapped up and damp, and add to it when the new
> stuff arrives. Using a mismatched plaster would be my
> least favorite choice, unless you are only using it for
> press-molds. For that, it would likely work fine.
>
> =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =
=3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3D
=3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0-Snail
>
--=3D20
http://twitter.com/MayLuk
http://www.takemehomeware.com/
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