Raku clay slip vs plaster/silica

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Ron Coleman
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Post by Ron Coleman »

Delores Taylor
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Re: Raku clay slip vs plaster/silica

Post by Delores Taylor »

[quote="Simon"]I've been experimenting with 50:50 hydrocal silica open face molds for filling with frit and fusing.

Simon molds will crack from being to wet or too dry are you using the manufacturers recommended plaster to water ratio? It isn't the size of your piece as that isn't that large of a mold. Maybe you have too wet a mix and it is structurally weak from a bad investment to water ratio.

Hydrocal will wick more water than #1 pottery plaster. Try using a 50/50 by weight then sprinkle into the water until you get a dry riverbed. Let slake, mix by hand (these are small molds and the drill whips up lost of air and causes bubbles). 125 to 350 soak 10 hrs 125 to 1000 soak 1 hr then go to process, etc. for the size of mold your working with.
Steph Mader
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Post by Steph Mader »

Hey Ron,

I also have read that there is chemically bound water that is released at a higher temp. I checked during the last firing to see if a couple pretty large molds were all the way dry yet. 12 hours into a dry cycle at 350 degrees, I held a piece of glass over the hole in the lid, and no fog. When the temp was ramping up, in the high 500's I held the glass there again out of curiousity, and it was really steaming again.

I think I'd heard originally that the chemically bound water released quite a bit higher than that, so it surprised me that the molds were drying further at such a low temp. By the time I was at 1000 it had stopped. I'll check again next week at various temps to see what's up with that.

I know you're right about the calcination happening at a lot lower temp. USGypsum even warns about calcination during dry cycles at too high a temp (temps as low as 150, if I'm remembering right), which makes me wonder about the wisdom of the typical 350 degree dry cycle. According to their charts, the higher the moisture content of the molds, the less susceptible (is there a spell-checker on this thing?) they are to calcination at drying temps.

Sorry to go on & on, but this is one of those things we should be able to understand theoretically, seems to me. I know that if I have a lot of detail in the mold, I don't dry it too much, as it gets really brittle and I start to have bits break off when the glass travels down into the void.

Oh, and my reasoning about the 900 hold is not just for the mold, but to get everything stabilized at the same temp before the glass starts to flow into the mold.

Steph
Delores Taylor
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Post by Delores Taylor »

lauri wrote:
Ron Coleman wrote: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
250 physical water is removed from the mold
350 chemically bound water is removed from the mold
1000 where there is a quartz inversion thing happening

These are the stress points for a mold and hold points classically I just ramp slow to 350 and have no problems then hold according to mold size and then go to 1000 and hold.

:idea: If you plan your firing right you get a nice toast work studio to enjoy for the day this is a definate perk but not quite as nice as going to visit Hawaii. David what's the temperature in your lovely land??????
Simon
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Re: Raku clay slip vs plaster/silica

Post by Simon »

I just wanted to post quickly to say that I am following this long thread closely and to thank all the various people for posting their experiences and advice. I have to say that I'm not a lot clearer on what will work for me. Partly because people's methods differ so much. I do have a lot of ideas to try though.

Mainly things are not advancing because our kiln is busy with other projects. This last week we have been firing several loads of my ceramics and today my wife is fusing glass sheets for her vases. We have a show coming up in 3 weeks so its not a good time for experimentation. My last silica/plaster mold that I made for her to test has sat neglected on her bench for over two weeks. I fear she lost confidence in my molds after my last two attempts cracked on her. :(

Nonetheless, it should be well dry by the time she uses it.
Nikki ONeill
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Post by Nikki ONeill »

Simon:
I have been following this thread closely too, and am so grateful to you for initiating it and for the considerable, thoughtful responses from the artists who have posted. Although my experience is limited, I've had great success with a 50/50 hydrocal/silica flour mold mix and BE frit and powder. The only failure has been when I picked up the wrong bag and accidentally mixed up 100% hydrocal :oops: Very small cracks sometimes appear, but they are acceptable. My molds are usually less than an inch thick, I fire after they are dry (sitting around 3-4 days), and vent the kiln until no moisture is visible on a stainless pan held at the crack.
Nikki.... so looking forward to meeting WGB exhibitors and seeing their work in person at the Smithsonian Craft Show tonight.
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