[Clayart] pottery plaster question

[Michael Wendt]

I always make a positive as Snail suggested because it makes it possible to 
cast new molds as needed.
The denser plasters like Hydrocal make strong master molds less prone to 
wear and chipping and require very little mold soap since they are not very 
porous.
Michael Wendt
----- Original Message ----- 
From: "Snail Scott" <claywork at flying-snail.com>
To: "Clayart international pottery discussion forum" 
<clayart at lists.clayartworld.com>
Sent: Monday, September 02, 2019 12:02 PM
Subject: Re: [Clayart] pottery plaster question



> On Sep 2, 2019, at 10:12 AM, Phyllis Canupp <pecanupp at gmail.com> wrote:
> ... I made a form and poured a plaster mold of the object...do not think 
> the plaster is thick enough. I thought about
> roughing up the surface and possibly drilling some 1/2" holes in the 
> bottom
> of the mold and adding more fresh (liquid) plaster…

If it is meant as a press mold, this will work. If it is meant for slip 
casting, having a discontinuity within the mold mass by adding another layer 
can sometimes result in not wicking the slip-water quite as well as if it 
were a single pour of plaster, resulting in a thinner casting on one side. 
This is usually a fairly subtle effect, though, and it may work OK anyway.

If you need to add plaster to an existing plaster form and have it hold 
together, it’s best do it almost immediately after pouring the first 
portion, before it cures. (And as you seem to know already, make the contact 
surface of the first portion rough.)  However, if you need to add plaster 
later, rough up the contact surface deeply, then saturate the cured plaster 
in water before adding the fresh plaster next to it. Otherwise, the dried 
cured plaster will draw water out of the fresh plaster and prevent it from 
curing properly. (Also true of concrete.)

I am assuming that the pattern is no longer available to pull a new mold 
part from? If you plan on doing a substantial amount of production form 
using this mold, consider using the present mold to making a new plaster 
pattern, then use that to make another (or several) new molds of this form. 
A ‘inter-negative’ plaster pattern will allow to to refine the form and to 
make fresh molds if it becomes necessary, and multiple molds will allow you 
to do simultaneous castings if you wish.

Snail Scott
claywork at flying-snail.com
www.snailscott.com