Ray Gonzalez on fri 28 sep 01
thinner plaster makes cleaner casts. i am wondering if you are using=
the
proper plaster to water ratio and if you are mixing it properly. i j=
ust
read a post (dont recall who) on how to mix and pour the plaster. if=
you
need further information email me offline at rayg@hughes.net and i wo=
uld be
happy to help you with this.
ray
----- Original Message -----
=46rom: "Nisse Holmstr=F6m"
To:
Sent: Friday, September 28, 2001 4:52 PM
Subject: Plaster & deflocculation
Tim Pozza wrote:
"Can a deflocculant be used with plaster?
Thinking about the last time I mixed a batch of plaster brings back t=
he
horror of the mess it made of the mixing bucket. It had started to
harden just as I was going to pour it.
If a defloc is used with plaster what would it be and how would the
material react? I thought it might lengthen or ruin curing."
Well, the closest I get to an answer would be the use of "retarders" =
and
"accelerators", ie additions that will either retard the curing of th=
e
plaster or accelerate it. Glue, gelatin, molasses, CMC, alcohol and v=
arious
phosphates will act as retarders - they will invariably affect the
curing-expansion and strength negatively, though. Calciumsulphate-dih=
ydrate
(=3D cured plaster, pulverized), potassium- and sodium-sulphate will
noticeably accelerate the curing.
Interesting effects can be achieved by mixing the gypsum with both a
retarder and an accelerator whereby the pooring-time is increased but=
the
curing-time maintained. Normal plasters will have a 1:3 relationship =
between
pouring and curing, ie 6:18, 8:24 or 10:30. With the addition of 0.7 =
%
potasiumsulphate + a retarder one can easily get a 20:30 relationship
(longer pouring-time but maintained curing).
A prolonged mixing-time will also result in an acceleration of the pl=
aster;
this will also make a stronger mould but the air-permeability of the
resulting mix will be noticeably decreased (a definite drawback in so=
me
applications).
Best of luck
/nisse/
http://w1.510.telia.com/%7eu51002415/
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=?iso-8859-1?Q?Nisse_Holmstr=F6m?= on sat 29 sep 01
Tim Pozza wrote:
"Can a deflocculant be used with plaster?
Thinking about the last time I mixed a batch of plaster brings back the
horror of the mess it made of the mixing bucket. It had started to
harden just as I was going to pour it.
If a defloc is used with plaster what would it be and how would the
material react? I thought it might lengthen or ruin curing."
Well, the closest I get to an answer would be the use of "retarders" and =
"accelerators", ie additions that will either retard the curing of the =
plaster or accelerate it. Glue, gelatin, molasses, CMC, alcohol and =
various phosphates will act as retarders - they will invariably affect =
the curing-expansion and strength negatively, though. =
Calciumsulphate-dihydrate (=3D cured plaster, pulverized), potassium- =
and sodium-sulphate will noticeably accelerate the curing.
=20
Interesting effects can be achieved by mixing the gypsum with both a =
retarder and an accelerator whereby the pooring-time is increased but =
the curing-time maintained. Normal plasters will have a 1:3 relationship =
between pouring and curing, ie 6:18, 8:24 or 10:30. With the addition of =
0.7 % potasiumsulphate + a retarder one can easily get a 20:30 =
relationship (longer pouring-time but maintained curing).
A prolonged mixing-time will also result in an acceleration of the =
plaster; this will also make a stronger mould but the air-permeability =
of the resulting mix will be noticeably decreased (a definite drawback =
in some applications).
Best of luck
/nisse/
http://w1.510.telia.com/%7eu51002415/
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