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GYPSUM,

I read in a lot of recipes that use an amount of Gypsum

This surely can't be Garden Store Variety Gypsum that is added to clay soil to help break it down a bit ??, surely
Or am I wrong and this is exactly what is being used ??

Thanks for any help

It is the same thing. You may see it in a brew shop labeled Calcium Sulphate. Adds hardness and lowers pH.
Edit: Or in craft shop as Plaster of Paris. Anything labeled as just Plaster, may be either gypsum, gypsum + lime or with cement added.

You can order food grade gypsum online for cheap. Don’t forget citric acid and pickling lime.

MarkChap wrote: ↑Tue Apr 29, 2025 1:28 am

This surely can't be Garden Store Variety Gypsum that is added to clay soil to help break it down

(Quoting lines from my old blog page from 5 yrs ago):

_ Calcium (Ca, element #20) is very common in the earth’s crust but since it is a reactive alkali earth metal, it is not found in a pure state. The pure metal of calcium was only isolated in 1808 by using electrolysis. The best sources of calcium normally come from rocks, crystals or minerals that are either carbonates or sulfates of calcium; like limestone, marble, travertine, chalk, gypsum, alabaster, caliche, marl and dolomite. Many of these rocks are sedimentary, having been formed at the bottom of ancient oceans, from skeletal debris left by marine organisms that were able to extract calcium from the seawater.
_ A pure chalk or pure limestone is called a “calcite” or “calcium carbonate” and has the formula CaCO3. “Calcination” is derived from a Roman word for the burning of lime. Calcination is the decomposition of calcium carbonate into calcium oxide (lime) and carbon dioxide [CaCO3 > + heat > CaO + CO2]. A calcine is the product of calcination (but the word “calcine” can also be used to describe just about any other mineral product being treated with heat to cause reduction, oxidization or to drive off water).
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_ The Egyptians some 5,000 years ago were were filling cracks and plastering the walls of Pharaoh’s tombs with a gypsum mortar made with calcined calcium sulfate.
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_Today’s drywall panels are created by sandwiching a core layer of wet gypsum plaster between heavy paper (or increasingly fiberglass surface sheets).  Gypsum of course is calcium sulfate (not calcium carbonate – lime) and the mineral is manifest in one of three levels of hydration.  Like lime, calcined calcium sulfate powder forms a mold-able paste upon hydration that hardens as crystalline calcium sulfate dihydrate and once hard does not easily dissolve again in contact with water.  The composition of drywall may differ between makers but the amount of gypsum ranges somewhere between 70 and 90%.  Gypsum is cheap to mine, fire-proof, and provides superior sound-deadening properties.  Besides gypsum, a sheet of wallboard might also contain wax, boric acid, a chelating compound, plasticizers, starch and obviously paper or fiberglass fibers.

_ It might be assumed that the joint compound used today to tape and seal large drywall panels is just liquefied gypsum.  Wall plaster at one time was predominately gypsum but today it is not.  Instead, most drywall mud is a putty made of slacked lime.  Another surprise ingredient finding its way into drywall mud is hydrated magnesium silicate.  This clay mineral better known as talc constitutes 5 to 15% of a given joint compound because its particles lie flat, help prevent cracking and sand to a smooth finish.  Besides talc, water and calcined limestone some other things hiding within drywall mud, might include a magnesium aluminium phyllosilicate clay (like attapulgite), expanded perlite and perhaps a thermoplastic polymer like ethylene-vinyl acetate.

_ Plaster of Paris was originally a high quality plaster made of calcium sulfate hemihydrate.  This quick set plaster gets its name from some ancient gypsum mines located in the Menilmontant hills of Paris. 

Thanks All,

I was mainly asking after seeing it used as a Yeast Nutrient in recipes not so much for ph control

Not a yeast nutrient. BUT it is a pH buffer, helps stabilize and lower mash pH for greater efficiency, and when used for beers it can help with water profile bringing out certain flavors perceptually.

After the mash, because it is a pH buffer to some extent, it CAN help yeast stay happy and have a good ferment. So it can have an impact on yeast, depending upon your water, recipe, and process.

I would not add it unless you have a thorough grasp on your water's chemistry.
You may not need it. It may do more harm than good, if you don't need it.