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

In which I try to make a pot with gypsum clay against everyone's advice and it didn't turn out like I expected.
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[@AncientPottery]

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

[@philipripper1522]

We all learn all the time! Thanks for trying new things and sharing with us!!!

[@ChadZuberAdventures]

That’s so interesting. I never knew anything about gypsum. I’m not sure that I’ve ever seen it before.

[@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

[@AncientPottery]

So short answer, this is the wrong kind of gypsum?

[@AncientPottery]

Or I got it too hot?

[@cortana2007]

@@AncientPottery you got it to hot, so it can no longer become the crystaline gypsum again, it will still disolve out of the pot (with water, in time) and leave holes, but it will not expand and destroy the pot

[@cortana2007]

@@AncientPottery it is the right kind of gypsum :D it is the normal kind, found in nature everywhere

[@AncientPottery]

@@cortana2007 cool thanks for the info

[@Kargoneth]

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.

[@Kargoneth]

Pre-hydrate such grout, then pack it into the hole. The additives, among other things, slows the reaction so that you can get the hydrated grout down into the borehole.

[@Kargoneth]

*slow down

[@AncientPottery]

I was expecting it too. Interesting idea, I had never thought of using the expansion of quicklime as a tool.

[@RainusBrainus]

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.

[@heathbryant3266]

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

[@josephjenson4894]

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

[@AncientPottery]

And now we know why the Sinagua potters never got into decorated pottery, they were just working hard to keep them from falling apart.

[@acavoxnegledajtelevizor401]

Try to use wet process but when clay settle down remove excess water and instead of just drying clay after that, try to add new fresh water and mix again change water 3-5 times like this and pls tell me if it work, salt from clay can be removed using this technique. I always experimenting with clay and In my area I haven't seen gipson in clay 😊

[@oldman6714]

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.

[@AncientPottery]

Element? I think don’t even remember saying that. I probably didn’t mean it in a chemistry sort of way.

[@oldman6714]

@@AncientPottery LOL, it was in the first 10 seconds of the video. I probably shouldn't have commented. It's just that sometimes it seemed that little comments like that could undo weeks of class.

[@rattheunloved9479]

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.

[@AncientPottery]

Yes that may be so. Also organic temper would do that, something like sawdust, charcoal or manure.

[@markgibsons_SWpottery]

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

[@AncientPottery]

Thanks Mark! Messy stuff for sure

[@BRANANDOOOM]

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.

[@AncientPottery]

Thanks. Yes some follow up might be worthwhile after some time and more research.

[@myth-termoth1621]

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.

[@AncientPottery]

Thanks for the info

[@fredericapanon207]

@myth-termoth1621, Ah-ha! So that's why using a double layer of gyprock panels will increase the fire rating of a wall.

[@pattheplanter]

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.

[@AncientPottery]

Thanks for the info. I had gypsum all wrong. I think it might be possible to wash it out, that should be the next experiment.

[@christineg8151]

@@AncientPottery Repeated washing is probably the way I'd go about it. Add enough water to let the clay settle to the bottom, allow it to leach for a few days, pour off, and repeat. Looks like gypsum has a solubility in water of 2.5 g/L so you need a pretty significant amount of water, even assuming it makes up a fairly low percentage of the clay, if you're going to prep a decent amount of clay.

[@wewenang5167]

@@AncientPottery yep i have seen potters used wet processing by washing it before using the clay...they even add some vinegar in the water during the process to get rid of the calcium.

[@GrannyGooseOnYouTube]

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

[@AncientPottery]

Thanks! Heading to California this week.

[@Adriaanthecrafter]

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

[@babystepsgarden6162]

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

[@AncientPottery]

You are so welcome!

[@catking1059]

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

[@AncientPottery]

Selenite is cool crystals. Sometimes when I am digging clay my wife goes around and collects selenite crystals, which is what I had plenty of them to use in this video.

[@JimmyCapricorn77]

I always heard that gypsum breaks down clay.

[@AncientPottery]

Well I will keep this pot around and see if anything else happens to it.

[@Francois_L_7933]

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

[@AncientPottery]

Thanks for the info. I like it on my walls but not in my pottery

[@myth-termoth1621]

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.

[@AncientPottery]

Yes definitely a viable solution. A lot of work but perhaps worth the effort for a good clay

[@MawoDuffer]

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.

[@angeladazlich7145]

That was an interesting experiment. Thanks!

[@AncientPottery]

You learn something new every day, well maybe you don't but I certainly do.

[@angeladazlich7145]

@@AncientPottery Dr. Seuss wrote, "The more I learn the more I learn how much I have to learn!" Since I learn slowly I'll be learning til I die :)

[@dezertdrifter]

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!

[@AncientPottery]

Thanks, the answer is I got it too hot, see my pinned comment above.

[@Partimepeasant]

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

[@AncientPottery]

Interesting, thanks

[@Sumermagor]

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....

[@ellenmortensen1987]

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

[@josephjenson4894]

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

[@AncientPottery]

That’s kind of what I expected to happen here.

[@AncientPottery]

Could it be calcium carbonate and not gypsum?

[@acavoxnegledajtelevizor401]

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

[@AncientPottery]

Yes, I think you are right depending on how fast those crystals can be dissolved, it should be able to be rinsed out or at least most of it.

[@GuyDoohan]

Great video 😃

[@AncientPottery]

Thanks! 😀

[@TheCommuted]

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

[@NayrbRellimer]

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.

[@GhostyOcean]

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?

[@AncientPottery]

I’m sure it does but I am not aware of any studies that have looked at this. Also keep in mind that different clays have different strengths, so the strength of the final pot will be the result of the temper and the clay.

[@mathiaslist6705]

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.

[@EXARCWithGrandpop]

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

[@AncientPottery]

Because it is water soluble I think levigation could remove some of the crystals but also would dissolve a lot of the crystals, so perhaps some combination of levigation and rinsing to remove the dissolved minerals. Similar to how Tony washes salt from his clay kzbin.info/www/bejne/o4DQiKabfJyHp7csi=alEjBabXns2NaUdW

[@jensenchavez265]

I use gypsum in the garden and for cultivating mushrooms, it's water soluble but not extremely so. It takes a while for it to dissolve, even if the water is hot or boiling.

[@pattheplanter]

@@jensenchavez265 Gypsum is less soluble in hotter water. 20°C (68°F) is the peak solubility. It is a bit unusual.

[@Kargoneth]

Heeere's Andy! Woo!

[@wcouch8]

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.

[@AncientPottery]

I think this is definitely a possibility.

[@ellenmortensen1987]

Waterglas is used as an additive in medicine, cosmetics.

[@suz4keeps]

Fascinating thanks

[@AncientPottery]

Very welcome

[@alexandrafinks6535]

I like how comfortable you are NOT wearing any shoes.

[@AncientPottery]

It was early in the morning and the temperature was perfect. You will notice when I opened the kiln later in the day I was wearing shoes because by then the pavement was HOT.

[@FlashyLight]

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

[@AncientPottery]

Unfortunately it’s hard for me to say since I have no experience with that clay. Maybe try a different clay if you can and see if it’s better.

[@xINVISIGOTHx]

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

[@ellenmortensen1987]

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.

[@AncientPottery]

Waterglass is good for sealing but isn't considered "food safe" so I guess it depends on what you plan on using the pot for.

[@tednelson5277]

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

[@AncientPottery]

Thanks, I guess I made a few assumptions about gypsum.

[@Timmyjg2004]

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?

[@socalboomer]

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

[@AncientPottery]

Yes, I think that may be doable. I need to do more experiments to say for sure

[@jakeedwards8974]

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

[@AncientPottery]

I made a cooking pot in this video. I think if I had it to do over again I would add handles to make it easy to move when hot. kzbin.info/www/bejne/hZPSq5RuiMuUkKssi=ZDETJNG2_XXD0MFm

[@FelixTheAnimator]

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

[@AncientPottery]

Yes that would work for porous pots

[@satinsteeldad]

Show me videos not so good this one actually very good

[@DimasFajar-ns4vb]

chair from pottery

[@jill-ti7oe]

😄👍👏