Making new fusing molds from clay

Stuart Clayman · Post by **Stuart Clayman** » Wed Aug 13, 2003 9:31 pm

Today I went to a potters studio and started making some molds. We used hig fire clay and now the clay is drying before it is fired. Tomorrow I will be making some more.

Any suggestions? I saw in another thread that it was recommended to put kiln wash on the piece before firing it. Any thoughts on this? The process is all new to me so any help or suggestions would be appreciated.

Stuart

Ron Coleman · Post by **Ron Coleman** » Wed Aug 13, 2003 10:34 pm

For the clay molds I've made, I waited till after the bisque firing to put the kilnwash on. Since a typical bisque firing will be in the neighborhood of 1800 f kilnwash won't be very effective after that temp and will have to be redone.

Drill any vent holes before firing the clay, much easier.

Very slow drying is the key to making clay molds that don't warp. Try some raku clay if warping during drying is a problem.

Ron

Stuart Clayman · Post by **Stuart Clayman** » Thu Aug 14, 2003 5:38 pm

Does putting kiln wash on before firing to Bisqu make the kiln wash permanent.. ie do not need to kiln wash again before slumping.

Also how mcuh kiln wash.. still 5 layers?

Suzan · Post by **Suzan** » Fri Aug 15, 2003 11:31 am

I'm not sure what applying kiln wash to greenware accomplishes. You know after firing something to full fuse once or twice that the kiln wash on your shelf needs to be replaced. At high fire clay temperatures, well above glass fusing temperatures, I doubt it would have any effectiveness. I believe there is a special technique where you slump glass into a greenware or "raw" mold, and this technique requires applying kiln wash to the greenware.

High fire clay is not necessary for glass slumping molds. Low fire earthenware, for example, fires at around Cone 04 which is in the 1900F range. This is well above the range you will be slumping any glass.

Also, I've read in two glass fusing books that clay molds for glass slumping should be fired two cones lower than what the clay manufacturer suggests. This makes the mold slightly less rigid and better able to handle the stresses of glass movement in repeated firings. I fire my clay molds at Cone 06, roughly 1800F.

Another important advantage of using low fire clay is that the lower temperature creates less wear and tear on your kiln's elements. High fire clays may be in the range of Cone 6 to Cone 10, which is above 2000F.

I'm afraid I can't remember Cone to temperature equivalences, but many pottery books have a table showing the cone to temperature equivalences. (My digital controller fires in both temperatures and cones, so I don't have to keep track

)

Cheers,
Suzan

charlie · Post by **charlie** » Fri Aug 15, 2003 11:39 am

what she said.

also remember that the clay will shrink. each clay body will shrink a different rate, so ask the pottery place how much.

Stuart Clayman · Post by **Stuart Clayman** » Fri Aug 15, 2003 1:05 pm

Luckily I am doing this at a potters studio with their kiln so I don't have to worry about the high temp in my kiln. They have suggested using the high fire clay because there is less chance of warpage.
As for shrinkage, this clay that we are using has a 10% shrinkage so I am taking that into consideration.
I still don't know the benefits of the kiln wash on the piece before the bisque firing. I have seen that recommended a few times. Like Susan said, I would think that it would burn off but maybe it helps to smooth out the piece or something.
Thanks for the info about firing the piece 2 cones less. The clay is for cones 4 to 8. Are you suggesting the lower end of the scale, or 2 cones lower?

Suzan · Post by **Suzan** » Sat Aug 16, 2003 12:41 pm

Yes, I am suggesting 2 cones lower, so in your case, cone 2 should be fine.

Cheers,
Suzan

Lauri Levanto · Post by **Lauri Levanto** » Sun Aug 17, 2003 9:03 am

Stuart:
Does putting kiln wash on before firing to Bisqu make the kiln wash permanent.. ie do not need to kiln wash again before slumping.

Also how mcuh kiln wash.. still 5 layers?

lauri: Hi, I'm the villain using kilnwash over greenware.
My thinking goes that clay has silica based materials that
love to react with glass. Tht's why kilnwash is used.
using it before bisquit firing has the advantage to get it
on a more porous absorbibg surface. The refractory part,
Al O2 becomes part of the innermost mold material.
To saturate the mold surface with refractory inhibits sticking.
I put one more layer after firing and touch up with dilute
kilnwash between fusings.

Suzan:
I'm not sure what applying kiln wash to greenware accomplishes. (...)peratures, I doubt it would have any effectiveness.
lauri: The refractory component of kiln wash remains intact
in a high fire. The kaolin tands to turn more reactive.
One reason kilnwash need to be replaced is that it seems to
absorb flux from the glass. This does not happen
when the empty mold is bisquit fired.

Suzan: High fire clay is not necessary for glass slumping molds. Low fire earthenware, for example, fires at around Cone 04 which is in the 1900F
range. This is well above the range you will be slumping any glass.
lauri: I agree with that, if your earthenware does not give
any discoloring oxides. In flowerpot melts the glass seems
to be clean. Maybe I am worried in vain.

Suzan: Also, I've read in two glass fusing books that clay molds for glass slumping should be fired two cones lower than what the clay manufacturer
suggests. This makes the mold slightly less rigid and better able to handle the stresses of glass movement in repeated firings. I fire my clay
molds at Cone 06, roughly 1800F.

lauri: I'm a sculptor, not a potter. Suspect my thoughts! Higher fired clay is sintered more and the object is stronger. Lower temp leaves the clay more porous and that
is good for molds.

Suzan: Another important advantage of using low fire clay is that the lower temperature creates less wear and tear on your kiln's elements. High fire
clays may be in the range of Cone 6 to Cone 10, which is above 2000F.
lauri: Keep in mind that the tems in ceramics are given
for a maturing process, just below the risk of meltdown.
The sintering starts in much lower temps, around 700-800 C.
The object is first loosely sintered, and higher turns more solid.
The temp given on the clay bag is the maturing temp, not
bisquit firing temp. So before 1000 C, say 1750 F gives
a good bisquit for high fire clays, too. Except pure porcellain
clay.

-lauri

dee · Post by **dee** » Sun Aug 17, 2003 12:26 pm

Ron Coleman wrote:For the clay molds I've made, I waited till after the bisque firing to put the kilnwash on. Since a typical bisque firing will be in the neighborhood of 1800 f kilnwash won't be very effective after that temp and will have to be redone.

Drill any vent holes before firing the clay, much easier.

Very slow drying is the key to making clay molds that don't warp. Try some raku clay if warping during drying is a problem.

Ron

ron - what type of clay are you using? i get confused by all the different types when i look at a pottery website and i've been wanting to do something involving custom molds...

thanks
D

Ron Coleman · Post by **Ron Coleman** » Sun Aug 17, 2003 2:23 pm

dee wrote:
ron - what type of clay are you using? i get confused by all the different types when i look at a pottery website and i've been wanting to do something involving custom molds...

thanks
D

I've used several types, regular white clay, terra cotta, and most recently raku clay. I think the raku is the best so far. It has a little less shrinkage than the others and doesn't warp as much during the drying process. There is a little texture from the grog in the raku clay.

I fired the raku pieces to only 1700 f and so far they have survived five slumpings. The raku clay doesn't weigh as much as the others either. The current pieces are oblong (6 x 20 inches) and were made from 5/16 inch thick slabs of clay.

Ron

Akua Lezli Hope · Post by **Akua Lezli Hope** » Sun Aug 17, 2003 4:56 pm

Ron Coleman wrote:
Drill any vent holes before firing the clay, much easier.
Ron

Awk! too late for me. (sigh)

I made some large, open face leaf molds of bisque-fired stoneware 18- 20" by 18--22" approx 1/2" thick....( mega rhubarb leaf castings)

Do open face molds have to be drilled too?
I'm also concerend that drilling will interrupt the surface texture.

thanks in advance for your thoughts on this.

Tom White · Post by **Tom White** » Sun Aug 17, 2003 8:35 pm

Akua, since you mentioned casting I do not think your mold needs to be drilled. When casting you are fully melting glass to a fluid state which conforms to the mold as it fills the mold. In most casting applications the glass is either in small pieces or dripped into the mold in a fluid state which fills low areas of the mold first instead of trapping bubbles there. OTOH when slumping into a mold the whole mold is covered with one sheet of glass which is only softened by heating so that it can conform to the shape of your mold. Any low areas in the mold which the softened glass might bridge before fully conforming to the mold have the potential to trap air between the sheet glass and the mold which can produce undesirable bubbles in the slumped item. The low areas need a small (1/16") hole at the bottom to allow the trapped air to escape through the mold to avoid the undesired bubbles. In general casting molds do not need to be drilled while there is a very good chance that slumping molds may need drilling to vent low areas of the mold.

Best wishes,
Tom in Texas

Akua Lezli Hope · Post by **Akua Lezli Hope** » Sun Aug 17, 2003 11:54 pm

Tom White wrote:Akua, since you mentioned casting I do not think your mold needs to be drilled.

O! I mentioned casting in terms of mold creation... not mold use...
I was trying to describe the mold... it is a leaf form and so ---more open than bowls for instance (into which I've slumped and had formed with holes)..

I've yet to use clay molds for kiln casting.... my intention for these clay molds are for fusing/slumping and I can't determine if the differential of a lower area is sufficient to require drilling..... perhaps I should post a picture?

Thanks,