Raku clay slip vs plaster/silica

[Simon]

I've been experimenting with 50:50 hydrocal silica open face molds for filling with frit and fusing. The molds have been about 6" by 6" by 3". So far the results have been unsatisfactory. The molds have cracked, sometimes catastrophically. when attempting to fire to cure, despite prolonged preliminary drying (days at 195F) and then slow firing to 1300. My last attempt includes adding grog to the mix at about 10% but that one is still drying so I don't know if it will succeed.

I was wondering whether it might be easier to pour molds using a raku clay slip. which would offer more resistance to thermal shock. The local ceramic supply place, Laguna Clay, offers a rather expensive slip intended for raku firings. Has anyone any experience with this, and if its more robust, is there any downside to using this material to pour molds? I can think of three without having used it:

1. Cost compared to plaster-silical
2. Longer drying time to get it solid enough to take out of the mold box
3. It is likely to be very tough when fired, so for casting type applications you couldn't break the mold apart to get the glass out. If you get an undercut in a homemade fusing/slumping mold you are in trouble?

Has anyone any tips on pouring slip molds?

[Tim Lewis]

Simon,

It is highly unlikely that the clay will be compatable enough to get it it survive. That would be enough for me to go back.

How do you mix the dry 50/50? It must be really well mixed so that the different sizes of the grains can "pack" (like various sizes of gravel). Also, rather than add grog try "Ludo" (pulverized & screened fired mold material). Many casters are finding fiberglass shorts (from a concrete supplier) stops the cracking and some are adding 5-10% kaolin and 3-5% to help with the sticking.

So the formula looks like this: 30/30/30 +5+3 or 30/30/30 +10+5

One of the problems with plaster is that it can loose its binding capacity before you reach glass melting temps so faster firing is often better.

What glass are you using?[/quote]

[watershed]

two other tips come to mind. Hydrocal may be too hard. Pottery plaster is fine.

Drying; I discovered a patient way to do it (find out tommorrow how well it works). I loaded the kiln, then suspended a box fan over the top, on low, for a day or so. According to smell, it worked pretty well.

You also might not have the plaster thick enough. And inch or 2 all the way around, and the chicken wire.

Not sure how advanced you are.

Greg

[Simon]

Simon,

It is highly unlikely that the clay will be compatable enough to get it it survive. That would be enough for me to go back.

Compatible? Do you mean it would break because of the expansion/contraction of the glass as it cools?

How do you mix the dry 50/50? It must be really well mixed so that the different sizes of the grains can "pack" (like various sizes of gravel).

I put the hydrocal and the silica in a plastic paint bucket with a piece of wood inside, sealed it and rolled it round for 10 mins. Let it settle and then added portions slowly to the desired volume of water, mixing constantly with a electric drill attachment until the consistency of thick cream and then poured.

Also, rather than add grog try "Ludo" (pulverized & screened fired mold material). Many casters are finding fiberglass shorts (from a concrete supplier) stops the cracking and some are adding 5-10% kaolin and 3-5% to help with the sticking.

I'll look into it. One reason I didn't want to do that was that I didn't want to compromise the smoothness of the mold by the glass fibers, or to have to pour a slip mixture and a separate outer mixture for a small mold. I was thinking the two layers of the mold might separate in firing.

One of the problems with plaster is that it can loose its binding capacity before you reach glass melting temps so faster firing is often better.

Interesting. What sort of ramp rate/firing schedule would you advocate for a smallish 6 x 6 x3 inch mold packed with frit. Say broken up Spectrum glass, though I haven't got that far often since the mold was breaking during the curing fire. That leads to another question. Is it better to just go direct to a fuse firing with an air dried mold, without a separate firing of the empty mold?

[watershed]

Yes, you've got it. Plaster silica is good for 1 count'em 1 heating and cooling cycle. Air dry, take it slow through the 180degF range (water burnoff) then you can speed up the firing. It can be tricky, but not quite rocket science. You will/may get cracks that give you flashing, esp if you go up to 1600 for the sepctrum.

Greg

[Jerry]

Well, I've been using a 50:50 #1 pottery plaster to silica mix for a long time and only rarely have cracking. My methods do vary from yours; here's what I do.

First, I pre mix my dry ingredients. In a sealed container large enough to hold them, I put a bag of each and mix them by hand over about an hour; it's dull but I get a "feel" for the lumps. When I actually use it, I use a hand sifter to fill a 16oz cup with and that's what goes into the water. Once I have islands forming over most of the surface, I wait several minutes and THEN do my mix. Now, I understand you using an electric mixer but I never have. Again, I do it by hand and that also helps me find the lumps and get rid of them.

Finally I pour, melt the wax out or pull out the positive and dry. That's the important part too because the only times I've ever had any cracking were the times I didn't get the mix dry. I like to let the mold set around the kiln room until it feels warm to the touch. If it's cool to the touch it's still wet. Then it's into the kiln at about 225. I get it to that temp and plan to keep it there until I can hold a mirror over one peep hole and not see condensation on the mirror. Typically I have to get the mold up to temp over several hours to avoid shock and hold it there several hours to allow it to stabalize before using the mirror.

I know, it's a pain to do all this but if you don't want cracks that's the way to go about it; your choice.

As far as the clay goes, once you get it dry, and you better be careful there, it will be quite hard and you will have the devil's own time getting your glass out of it if there's even the smallest undercut. This is experience talking here; I'm married to a potter.

Good luck.

Jerry

[Tim Lewis]

Something I missed in your first post was the hydrocal. It is a type 2 plaster and is more vulerable to cracking at these temps. It might be ok on the outside of the mold but the inside needs to be softer.

Yes, I mean that the clay will break. You need a binder (#1 Pottery plaster) a refractory (silica, 200 mesh), perhaps something to help with the heat/moisture transfer (ludo or maybe grog), and maybe something to help with the glass sticking. Also, Ihave never heard complains about the fiberglass sticking since it has a higher melt point than the soft stuff we use.

I am buying an Odjob cement mixer so that I can seal it and kick it around to mix after I use my paint mixer. An automatic mixer would be best but this will work for now since it has interior baffles.

If you are able to make the inside "splash coat" at the same time and put the main coat on just after the first the seperation should be minimal.

I have been reading a CD from some research on kiln casting and some are filling the mold wet and then:
fire it to 400 F over 5 hours and soak for 4 hours,
ramp to 1100 over 4 hours and soak 4 hours,
fire to top temp (15 something) in 2 hours and soak 1 hr or so
crash to above anneal

top temp can have longer soak depending on how much fining out is needed

Another one is strait to 1530 or so over 8.5 hours. I presume the mold was dried first.

Basically the "curing fire" is not necessary according to these experienced casters and the researchers. This way of firing is new to me so I am trying it for the first time this week. I will know the results in by the end of the week. I suspect there are others on this bbs that do it that way.

[Steph Mader]

I wonder if the molds are getting thermal shocked at the end of the dry cycle, or if the dry cycle temp is too high. If the mold is dried too hot at the beginning of the cycle, then the steam can blow cracks in the mold. I think Carol mentioned 225 or so.

I dry the molds for a week, then load them in the kiln. I go slow up to 250, dry 12 hours, then up to 350 for another 12 hours. The molds are pretty big, though - 12" in diameter and about 12" tall. I also hold for about 4 hours at between 800 & 900, to drive out the molecular moisture before going up to melt temp.

There are a few other things to check - is the hydrocal old, is there not enough dry mix in the mix:water ratio, was it mixed too short a time (5-ish minutes), is the part of the mold that contacts the kiln shelf uneven so it puts too much stress on the mold, is the mold too close to the elements?

Those are my thoughts, anyway.

Hey Tim, where did you get the CD?

Steph

[Patty Gray]

there are also two new mold mixtures for doing casting. One is Gerry Newcomb's and the other is Michael Dupille's. Both are outstanding. Both mold mixtures are available through NW Art Glass and I believe Clay Art Center. Both have wonderful smooth textures, etc.

About drying your molds, there are several different mold drying stations you can build. You may want to contact Gerry or Michael and find out what they suggest.

I took a wonderful class from Delores Taylor recently and the mention of hand mixing your mixture is very important. Taking your hand into the mixture and bringing the lumps to the surface carefully you can eliminate the lumps and also eliminate lots of bubbles.

You know, I would at least try the raku clay in a small experiment. I did a casting class last year (in fact it was a year ago this time). You can do wonders with this clay. We did the casting molds intended only for raku but it may work with glass (just remember to do the release for this, otherwise you may have a glass piece that is one with the mold, lol).

Let us know how you proceed with this, and I wish you the best of luck.

Patty
time4sun@worldnet.att.net
http://www.pattygray.com

[Nikki ONeill]

Tim: I bought the CD too and found it very interesting. I had trouble, tho, seeing or printing the text of blocks in some of the table. Did it work for you?
The CD is titled "Mixing With the Best. Investigation and Comparison of Contemporary Working Methods and Mould Making Materials For Use in the Kiln Forming of Glass" It was written and edited by Angela Thwaites and published though the Royal College of Art in London. It's in an Acrobat PDF file, and advertised in Glass magazine. I bought it by e-mailing Sandra Reynard at s.reynard@ca.ac.uk.
Nikki

[Ron Coleman]

I dry the molds for a week, then load them in the kiln. I go slow up to 250, dry 12 hours, then up to 350 for another 12 hours. The molds are pretty big, though - 12" in diameter and about 12" tall. I also hold for about 4 hours at between 800 & 900, to drive out the molecular moisture before going up to melt temp.

Steph

Just curious why you soak at 800-900 to drive off the bound water? The calcining temperature for gypsum plaster is about 325 f. Above that all the water is gone.

Brad's book mentions 325 f as a holding point and Fenton and Kerwin mention the same temperature in their book. Nowhere can I find a reason for holding at a higher temperature.

Ron

[Carol Craiglow]

Here's a link to a great website with some info on using clay for casting. It's all Stina uses. She experimented extensively during her master's studies at CA College of ARts and Crafts. Right now she's doing small shapes but has successfully cast larger pieces as well. I didn't find the link to the page where she describes her technique in detail, I believe it's under the FAQ page if I remember correctly:

http://www.art.net/Studios/Sculptors/Stina/

I have the Royal College CD and have found it interesting. It costs about USD 28. I think it's just a good resource and overview of some European casting techniques.

Carol

[charlie holden]

Just curious why you soak at 800-900 to drive off the bound water? The calcining temperature for gypsum plaster is about 325 f. Above that all the water is gone.

Brad's book mentions 325 f as a holding point and Fenton and Kerwin mention the same temperature in their book. Nowhere can I find a reason for holding at a higher temperature.

Ron

I don't know what the calcining temperature means. But I've been told, and it seems I've read but I can't remember where, that there is some chemically bound water that is released around 1000 F. It's one of those things I've always done, so I can't say if it would make any difference if I skipped it.

You should definitely pass slowly through 1000 to 1100 because of the quartz inversion point. Somewhere in there the silica changes its structure and expands dramatically.

Quartz inversion is also a good reason to thoroughly mix your dry ingrediants. If you have pockets of mostly silica they will expand more than the material around them and split the mold. I mix my dry plaster/silica for more than 45 minutes.

As to Simon's original mixing technique, you should not stir the water at all until all the plaster/silica has been added and it has slaked, (sat there unmolested) for about 5 minutes. All the plaster needs to get equally wet or it will harden unevenly. I know this is nerve wracking but you can actually let it slake for 15 minutes or more. It is the stirring that sets it off. So if you stir from the start the mix will be hardening while you are still adding dry material.

I would also suggest weighing your dry materials and mixing by consistant ratio to your water weight. For a hard, strong mold I mix at a ratio of 1:1.8 -- water to dry mix. This will seem like a huge amount of dry mix when you first do it, but let it slake and it will all get wet. For a softer mold, (for the inside of hollow form molds) I use 1:1.7.

I disagree somewhat with Tim. I know that lots of people add ludo or grog or other things. But I think it is dangerous to get the amount of plaster too far below 50% by weight unless there is some other binder, like cement. Personally, I stick mostly to 50/50 plaster silica except for face coats, where I reduce the amount of silica but add zircopax to get a better release from the glass. Zircopax is zircon bound to silica.

[Ron Coleman]

Thanks Charlie for the info on mixing. I didn't know you could let the wet components sit for that long without it going off.

The calcining temperature is the point they heat crushed gypsum rock to make the plaster in the first place. That's the point that all of the bound water is driven off and you have plaster. Just add water and it goes back the other way.

Ron

[Tim Lewis]

Here are 3 pages from Stina's big web site. I had completely forgotten about her clay molds. She uses kiln wash to prevent siticking and doesn't worry so much about the fine surface many of us are after out of the mold. She uses high fire stoneware and bisque fires the mold first.

http://www.art.net/Studios/Sculptors/St ... aking.html
http://www.art.net/Studios/Sculptors/St ... molds.html
http://www.art.net/Studios/Sculptors/Stina/FAQ.html

Charlie, I am reporting what people told Angela Thwaites and what her conclusions were from the mixing and firing she did for this CD. I am in the process of trying her recipes and will report my findings. By the way, she uses 1:1.4 ratio with the 30/30/30 mix. It might make a difference.

[Carol Craiglow]

Just to offer a slightly different approach...I do not dry mix my ingredients before adding them to water. I take one scoop of plaster and one scoop of silica (I usually put a little more plaster than silica per scoop to approximate by weight) and put in a bucket. I mix only enough for the mold or molds I will be making at one time. I allow it to slake for 2 minutes, then either use an electric mixer for 2 minutes or mix by hand for whatever time it takes for the mix to coat my hand with a consistency like cream.

I know others who do very large castings and use this same approach. I think it works because I only mix one batch at a time, so I know that the total amount of plaster and silica are right on. The only time I had a problem was once when the water mix was too warm and the mix set up too quickly, and then the ingredients didn't mix properly. FYI to folks just starting this, the plaster should be about 70F ideally.

I can appreciate the dry mixing approach, but I don't have the patience for it and this system works for me. Also, I wonder if you let the ingredients sit for a while if they might tend to segregate again. I know when I've purchased Mizzou refractory material, the manufacturer says clearly that you need to remix the bags because some of the more dense ingredients tend to settle out.

Just another approach...I think everyone experiments and finds what works for them.

Carol

[Lauri Levanto]

Thanks Charlie for the info on mixing. I didn't know you could let the wet components sit for that long without it going off.

The calcining temperature is the point they heat crushed gypsum rock to make the plaster in the first place. That's the point that all of the bound water is driven off and you have plaster. Just add water and it goes back the other way.

Ron

From myarchives i found:
1100 F= 593 C soak to get chemically bonded water out m Charliehol 15.10.2002

In sime potteru texts I have found similar info.
For plaster the calcining range is from 42 C to 220(?) C.
That is the reversible process that drives half of
the water out ( and can be reversed). The temp
Charliehol mentioned removes the rest of bonded water,
and the product no longer hardens with water.
Try with old moldmix!

-lauri

[Tim Lewis]

Carol, It would help me understand how you do this without the advantage of dry mixing if I knew the answers to the follwing:

How thick are your molds? Are they poured or hand built?
Do your molds crack (resulting in flashing) or does the glass stick?
What glass are you using?

Thanks,

[Carol Craiglow]

Hi Tim...I do both poured and hand-built molds, but predominantly poured.

Most of my shapes are openfaced, but I have done 3D lay-up and poured molds this way as well, with no flashing. The molds average between 1 and 2 inches thick, but I've made them up to 3 inches thick with fiberglass in the outer coat.

I only get flashing if I heat up too fast...in my experience that is the primary factor in flashing.

I don't think he'd mind me using his name...but this is the way Mark Abilldgaard makes his molds; we've discussed it. He makes very large (up to 3 ft long) open-faced cast pieces.

As I say, I am not arguing that this is "the" way to make molds, there are many paths! I only mentioned this as one approach to mold making that works well for me.

Carol

[Tim Lewis]

Enquiring minds...

I only get flashing if I heat up too fast...in my experience that is the primary factor in flashing.

How fast do you fire and is it wet or mostly dried by that time? Your dry climate can certainly be a factor here.

Since I got the CD I have been questioning everything I thought I knew about casting and when you described your technique it is almost the opposite of what they say about the importance of "packing." Essentially you avoid that issue and your work turns out fine. From the colors of your pieces I assume you are using BE or Uro so you have to fire fairly high. I have had flashing problems but I suspect it is because I fired too slowly plus I am using a closed mold with some possible weight problems.