I Was Wrong About Gypsum In Clay, But You Should Still Avoid It 

Thanks to a viewer who knows a lot more about chemistry than I do, I have the answer. In order to turn gypsum into plaster you need to get between 120 and 180 C, a pretty narrow window. Because I went well over 180 this gypsum will not expand or turn hard. So this would never be an issue in pottery since you can't make pottery at those low temps. Now I know, thanks @cortana2007

gypsum is CaSO4 * 2 H2O, at 110 - 120 C it loos 3/2 H2O remaining CaSO4 *1/2 H2O, if you heat it over 180C it will become anhydrous CaSO4. Mixed with water CaSO4 *1/2 H2O will rehydrate to CaSO4 * 2 H2O and crystallize, harden and expand, anhydrous CaSO4 will no longer rehydrate so it will not harden any more and will not form any crystals. But both forms are somewhat soluble in water

Adorable test pot. That selenite chunk definitely changed forms. I was expecting spalling. Yes. Calcined calcium carbonate (quicklime) expands very forcefully when hydrated. It is also causic and the reaction is highly exothermic. It can split rocks when packed into boreholes while dry and then hydrated (e.g. demolition grout, which is primarily quicklime, but also has some other additives for better performance when used for this task). A useful tool when you lack dynamiite.

We use the selenite clay for hardware pottery only. Nothing decorative for sure! Great info. Thanks, Andy!

Thank you sooooo much! This is exactly how the majority of clay in the verde valley is. I finally found some deposits without it but the prettiest clays are all full of gypsum

It takes a large amount of water to dissolve a small amount of gypsum, and fully cooked anhydrous gypsum dissolves so slowly that it is insoluble for all practical purposes. Plaster makes great drywall because in a fire it absorbs a great deal of heat turning anhydrous which seriously discourages a fire from spreading to the next room.

I can't help but wonder if the extra porous clay body might be really nice for those garden ollas people make. Something just a little more porous for extra-thirsty plants.

LOL, the retired chemistry teacher in my groaned when you called gypsum an element. I'll probably never make a single pot, but enjoy your videos.

this guy is loving and caring say you're the best if you agree

That's actually good to know if you are trying to make porous clay. You could use it to make clay balls for hydroponics and such. In a primitive setting it could be useful for making "filter" pots for water purification.

I always wanted to learn how to make pottery. Ever since I can remember. Thank you for showing us how. ❤️😀❤️

I really love your experimental videos. It's fun to learn alongside you. Thanks for this.

Thanks for the video. One thing I have learned to avoid is silt as it causes small crackles in clay.

I have seen 800 yr old native pottery around here in Illinois with gypsum in it. Seems it didn't bother the natives.

I always heard that gypsum breaks down clay.

That was an interesting experiment. Thanks!

Curious. I wonder if a little teeney tiny bit of gypsum in clay when making an olla will make it more pourous in a good way. I feel an experiment developing.

Would levagation help to separate this material out of the clay, since it is water soluble?

waterglass is approved as a detergent for removing grease from vegetable/ plant and oil from drilling.

Great video 😃

I had a commercial pot that had white precipitates which crumbled after several years

Heeere's Andy! Woo!

Beautiful red color. Looks like it just make your pot more porous you can try to make covershird out of this clay to see will it last longer than covershirds from normal clay porous material should be more thermal shock resistant. If this gipson material water soluble it can be mostly removed using wet process and changing water in clay few times

I like how comfortable you are NOT wearing any shoes.

Waterglas is used as an additive in medicine, cosmetics.

Also you could levigate and then flocculate the clay to drop out the minerals and big crystals. You would probably need more temper in it, but you would be able to drop out all the non clay particles.

OLLAS. I need some ollas for my garden made out of that clay.

Gypsum will not react with acids and therefore will not fizz.

Hey Andy! I had a question who has more experience with pottery. Does adding temper to clay make the final ceramic mechanically weaker? In other words, does the final ceramic's strength have a negative correlation with the amount of temper used?

What does having only like 3 micro hairline cracks in my pots mean? I constantly only get a few super small hairline cracks but don’t know the cause?

wonder if you could wash out the gypsum / carbonate leaving just the good clay?

Show me videos not so good this one actually very good

Hi Andy I've been trying to make coil pinched pots with commercial Stoneware clay and bringing them to narrow the pot but I'm struggling a lot the clay keeps breaking, doesn't look smooth and is not coming in right, is this because of the clay I'm using? It's called k9 Stoneware clay

chair from pottery

I wonder if you sealed that pot with wax or by seasoning with a fat, if that would seal off the gypsum and keep it from dissolving out of the pot? A part 2 to this episode perhaps? Lol great video Andy, thanks for everything you do and share.

You only get fizz from carbonates, not from all calcium salts. The fizz is the carbon dioxide released when a stronger acid is applied. Vinegar is not a stronger acid than sulphuric, so won't replace sulphates. Can you just wash the gypsum out before using the clay? Or wash it out after firing to get some interesting porosity.

Gypsum is actually Calcium Sulfate. It's also the stuff used in drywall...

can you make a pot out of that cooked gypsum mud and fire it

I really need to get you some of this camp verde gypsum clay, every pot I have made with it fizzles and then eventually falls apart. I would love to see if you have the same problems

I find this very interesting. I have a crystal collection and my selenite is my favorite piece. I am a sculptor and I have been learning about pottery for years. I don’t know how many of my friends who also like crystals know that gypsum is selenite

😄👍👏

Thanks for your videos. I was able to pit fire some clay marbles. And it was clay I was able to find in my own area.

I'm amazed your clay was still plastic when you shaped it before firing. Gypsum is often added to clay-rich soil to improve drainage and prevent clay particles from sticking together. The only other potter I've seen who works with gypsum-rich clay is Tony Soares who uses a source of clay from the Salton Sea basin. I'm not certain if his wet-processing method makes any difference to his clay plasticity any more after seeing this video.

Ima beginner in making pottery what is a simple pot that I can make for cooking?

Chemical removal? The only reaction I found is reduction by firing gypsum with coal to get calcium carbonate and then making that water soluble with vinegar to get calcium acetate which can be washed out. Not sure about the temperatures required but that should have happened to some degree with the gypsum in there.

Dear @AncientPottery I have a problem with my local wild clay that is very fine, it is a pale grey clay, the area is known for moderate sodic soils with calcium based sedimentary rocks , and kaolin is also found in the area, but this clay is grey, the clay is very good to work with, it doesnt crack when it dries or anything, it seems to have a high amount microscopic mica and silica particles its glittering the area is sandy aswell but the sand has been washed out , so the main problem is that i tried bisque firing it to 1000c to 1140c and it does not vitrify, it doesnt harden at all, and it just turns back to powder when pressed together easily with fingers after a bisque firing, i have a feeling this is still good clay but i dont know how to amend it....

What a fun experiment. I always thought it would react when wet. Maybe the firing changed the composition so it doesn’t react when wet? Great video!

if you let your pot soak waterglass, the waterglass will dissolve the gypsum making it in to Na2SO4. The waterglass will waterproof your pot. It might take some time for the waterglass to harden up. It takes CO2, which is why it takes time.

Fascinating thanks