Calcium Sulfate / Gypsum

Where can I buy a pound or two of this and not have the shipping be crazy? Where could I buy it in say a local store? I have all the other chemicals for my hydroponics except this one and I don't want to have to buy a ton.
MJ
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mj wrote:

Plaster of Paris. You'll probably have to hydrate it to make it useful.
Bob
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As in every elementary school project plaster of Paris? I need it in a dry form mixed with all the other stuff and then it is dissolved into the water system.
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mj wrote:

Yes. But you want hydrated calcium sulfate, CaSO42(H20), and Plaster of Paris has been dehydrated to 2(CaSO4)H2O. So you'll probably want to add excess water, let it harden, then pulverize it.
Bob
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I failed chemistry, why harden it first, because just adding it to the water in the system it will harden there? Thanks for all the help
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mj wrote:

I assume if you mix Plaster of Paris with the other stuff dry, the other salts are hygroscopic and will absorb moisture from the air and everything will clump up into little rocks. You haven't given enough details about what you're planning to really know -- all I told you was how to get the cheap gypsum. :-)
(You know Calcium Sulfate is not very soluble in water?)
Even cheaper would to tear the paper off a scrap of Sheetrock and use use the white stuff inside, but it might not be pure enough for you.
Bob
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mj wrote:

Please consider that this is an international forum. How is anybody going to answer your question as asked without knowing where you are?
You might also consider how pure the calcium sulphate needs to be. The stuff sold for agricultural use often has small amounts of heavy metals in it. I am not sure what the consequences of that would be in hydroponics. You will pay much more per kilo for purified stuff than standard.
Thirdly consider why you are adding calcium sulphate. Is it to provide mineral calcium or sulphur? There are some obvious substitutes for providing calcium possibly some for sulphur if given some thought. There may be side effects of substitution so if you don't follow the chemistry involved you will have to stick to the formula you have been given.
David
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On 11/8/10 10:30 AM, mj wrote:

Many nurseries sell gypsum in small bags (5 or 10 pounds). But gypsum is quite cheap. I think I paid less than $10 for a 50-pound sack at Orchard Supply (OSH). But then, I use about 50 pounds a year to keep my clay soil porous. I may use 100 pounds this winter, broadcasting it over my entire front and back yards; it's been at least two years since I did that.
--
David E. Ross
Climate: California Mediterranean
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