John Rodgers on fri 18 jul 08
Hi Nancy,
I do a lot of slip casting. Here are a few pointers.
Go to the Laguna Clay website and download their instructions on how to
mix slip. Their procedures pretty much cover most slips, no matter the
formula. Particularly the ratio of water to clay. Once you have the clay
mixed into the water you will want to adjust the density and the viscosity.
To test the density weight a pint of water and a pint of slip.Divide the
weight of the water into the weight of the slip. You will get a number
like 1.5 or 1.8 or something similar. If the number is less than 1.75
you don't have enough clay in the mix. It it is above 1.85 you have to
much clay in the mix so you need to add some water. The best numbers to
work with is that range anywhere between 1.75 and 1.85. I usually start
at 1.75 and as I do casting, the water loss in the reclaimed clay will
slowly change the number to 1.85, then I add some water.
To test the viscosity you need a viscosity cup. I strongly recommend the
one that new Mexico Clay sells that is really a paint tester. It only
costs $4.95 and does an excellent job, is easy to clean up, and is very
fast. The old business of using a two-hole stopper, an Erlenmeyer flask
and glass tubing is bogus - takes forever and they charge like $25 for
glass and more for plastic. Use the New Mexico Clay viscosity cup. It
has a tiny hole in the bottom, and if the slip is adjusted properly the
cup filled with slip will drain in 23 seconds. Slower than that and you
need to add some Sodium Silicate. You have to be careful with any
adjustments because just the right amount will make the clay flow very
easily. To much and it will become more viscous and won't drain, or will
do so very slowly. Once you have over done it with silicate, you must
add more clay and re-adjust the density, then re- adjust he silicate.
If you were to plot viscosity vs silicate it would plot in a graph
shaped like a "U" As you add silicate, the viscosity decreases and the
flow rate goes up. At Exactly the right amount. all the numbers will be
at the very bottom of the "U". As more silicate is added, the viscosity
increases, the flow rate slows, and the numbers move up the leg of the
"U". When that happens you are in trouble. Big adjustments in the amount
of clay have to be made.
So - First- Adjust the density. Second - adjust the viscosity. - Always
in that order.
Regards mixing your slip, a power mixer is recommended, and a long
mixing period for fresh slip is recommended. I finally went so far as to
buy a 33 gallon slip mixing tank. Best investment I have made in a long
time. Makes it so easy to mix slip, recover reclaimed clay, and adjust
the density and viscosity. I recommend it.
Hope this helps.
Good luck.
Regards,
John Rodgers
Chelsea, AL
Nancy Udell wrote:
> Hi clay buds,
>
> Requesting help from anyone who has knowledge of slip casting and slip
> formulation. Decided to try as a summer project making a set of slip
> cast dinnerware. I am fairly familiar with plaster, but slip is new
> to me. Bought Andrew Martin’s book and made a couple test molds. So
> far so good.
>
> Mixed up a test batch of Hensley/Polseno Cone 7 Slip
>
> OM4 25
> EPK 18
> Tile 6 5
> Neph Sy 35
> Custer 9
> Frit 3110 1
> Flint 7
>
> 100
> water 42
> sodium sil .3
>
> I ended up with a slip that will make castings, but does not release
> from the mold uniformly – sometimes sticks and cracks – and does not
> dry and become solid in a short period of time – 3 or 4 hours or
> overnight. The book says that for troubleshooting these problems
> could be too much water, too much EPK or not enough defloculant.
>
> A couple questions:
>
> 1. Am I adding the right amount of water and sodium sil?
>
> I assumed that because the dry ingredients are by weight, that the
> water and sodium sil should also be by weight. The book (while
> excellent in many respects) does not make this clear anywhere. I found
> the weight of water on the internet, and since water is 42, added 19
> litres (42 pounds). I then converted .3 lbs to grams and weighed out
> 113.4 gms of sodium sil on my gram scale.
>
> Is this the right way to do it?
>
> If this is right (i.e. if I have correctly mixed the slip) how do I
> know which of the trouble shooting problems to try to correct? I
> don’t know how to do this with multiple variables.
>
> We are at 7200 feet altitude here. Can that make a difference?
>
> Thanks for any intelligence you may be able to share.
>
> Nancy Udell
> Santa Fe, New Mexico
> 505.984.9907
> www.clayandcolor.net
>
>
Nancy Udell on fri 18 jul 08
Hi clay buds,
Requesting help from anyone who has knowledge of slip casting and =20
slip formulation. Decided to try as a summer project making a set of =20=
slip cast dinnerware. I am fairly familiar with plaster, but slip is =20=
new to me. Bought Andrew Martin=92s book and made a couple test =20
molds. So far so good.
Mixed up a test batch of Hensley/Polseno Cone 7 Slip
OM4 25
EPK 18
Tile 6 5
Neph Sy 35
Custer 9
Frit 3110 1
Flint 7
100
water 42
sodium sil .3
I ended up with a slip that will make castings, but does not release =20
from the mold uniformly =96 sometimes sticks and cracks =96 and does not =
=20
dry and become solid in a short period of time =96 3 or 4 hours or =20
overnight. The book says that for troubleshooting these problems =20
could be too much water, too much EPK or not enough defloculant.
A couple questions:
1. Am I adding the right amount of water and sodium sil?
I assumed that because the dry ingredients are by weight, that the =20
water and sodium sil should also be by weight. The book (while =20
excellent in many respects) does not make this clear anywhere. I =20
found the weight of water on the internet, and since water is 42, =20
added 19 litres (42 pounds). I then converted .3 lbs to grams and =20
weighed out 113.4 gms of sodium sil on my gram scale.
Is this the right way to do it?
If this is right (i.e. if I have correctly mixed the slip) how do I =20
know which of the trouble shooting problems to try to correct? I =20
don=92t know how to do this with multiple variables.
We are at 7200 feet altitude here. Can that make a difference?
Thanks for any intelligence you may be able to share.
Nancy Udell
Santa Fe, New Mexico
505.984.9907
www.clayandcolor.net
James F on sat 19 jul 08
>=20
> Requesting help from anyone who has knowledge of slip casting and =20
> slip formulation.=20
these problems =20
> could be too much water=2C too much EPK or not enough defloculant.
>=20
> A couple questions:
>=20
> 1. Am I adding the right amount of water and sodium sil?
>=20
> I assumed that because the dry ingredients are by weight=2C that the =20
> water and sodium sil should also be by weight. The book (while =20
> excellent in many respects) does not make this clear anywhere. I =20
> found the weight of water on the internet=2C and since water is 42=2C =20
> added 19 litres (42 pounds). I then converted .3 lbs to grams and =20
> weighed out 113.4 gms of sodium sil on my gram scale.
>=20
> Is this the right way to do it?
>=20
>
Nancy...
I am far from expert in slip casting=2C but had a similar problem re sodium=
silicate. I went round and round with the technical support people at the=
manufacturer=2C and STILL could not get a straight answer.
Here is the problem: Sodium silicate is available as a dry powder or as a =
liquid. The dry powder could=2C of course=2C be weighed out accurately. H=
owever=2C the dry powder has a very short shelf life=2C and converts to sod=
a ash and silicic acid from contact with air (CO2=2C I believe)=2C so no lo=
nger functions in quite the same way. The liquid form is stable=2C but is =
only a 34% solution. It seemed to me=2C therefore=2C that one would have t=
o weigh out nearly three times as much liquid (X / .34) to obtain the requi=
site amount of actual sodium silicate called for. This is the issue upon w=
hich I could get no straight answer from the manufacturer nor my supplier.
Ultimately=2C I still do not know if I am correct in that one must use X / =
.34 of liquid to equal X dry powder. Any science-types have ideas?
...James
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