Talc-based body holding water (or rather not)

[Nini]

Hello everyone, 

Longtime lurker, first time poster  

I have a bit of a strange question, or rather a desperate plea for help. For the last year or so I’ve been experimenting with a talc based clay body (fired to cone 6). It’s an odd one, but has some very interesting properties and I love working with it. The only problem is I can’t make it hold water for the life of me, so I’m hoping someone knowledgeable might have some ideas.  I mostly do sculptural work but I would love to be able to make a vase or plant pot from this clay. 

The recipe is 60-65% talc, and as for the rest I’ve tried a few versions but let’s say 60 talc, 20 kaolin, 10 silica, 10 ball clay. Have also done variations including cornish stone, wollastonite, alumina etc, keeping the talc content at least at 60, and kaolin pretty substantial. 

It has some unusual responses to glaze, which is partly what I find so fascinating about it. Most gloss / satin glazes will cause the body to crack, while some of them just turn matte. Funnily enough the only gloss glaze which fits is Britt’s snowflake crackle, it doesn’t crackle at all on this clay. I have so far achieved one single tiny test pot using the snowflake glaze which does hold water. It was very exciting! But the problem is, if there is so much as a single pinhole, water will go right through it. Reglazing doesn’t necessarily help either. 

I’m desperate to find some way to make it watertight. I don’t suppose with that amount of talc there’d be any way to vitrify it… I wonder if there are any engobes or vitreous slips that might do the trick? I’ve even heard of an old technique of “curing” earthenware by soaking it in boiling milk, which I’m starting to wonder about! At this stage any and all ideas welcome and greatly appreciated

[Babs]

Not able to advise on the clay body but the curing with milk is no cure. Better use a tile sealer on inside and only for vases , not foodware, if no one can sort your problem.

I thought talc at high temp becomes a flux but your clay doesnt sound like its vitrifying,  and talc used in clay bodies more in the raku temp range of firing bcause of its ability to withstand thermal shock. Is your work really thin?  c.o.e of glazes you are using and your body are incompatible.

[Nini]

Hi Babs, thank you for your advice. Yes I’m starting to think that sealing the interior after firing might be the best solution… I wanted to avoid this and keep it purely in the ceramic realm but I suppose as a back up option for the ones that still leak after glazing it wouldn’t be the worst pottery sin!!  I don’t make any tableware from this, it would be only vases or plant pots. 

I think safe to say I’m not using talc in the traditional (or maybe even sensible) way! It was some years ago that I started playing around with low fire talc bodies, and then I tried firing it to cone 6 and found that some glazes looked really interesting on it, so I kept exploring. What I’ve been making from this body is not thin at all, in fact the opposite - because it’s so light and marshmallow-y I can get super thick sections without the piece cracking or weighing a tonne, and I enjoy the look of that. I basically make big rocks… would just love to make a hollow one I can put a plant in  

COE is not something I’ve fully wrapped my brain around yet but yes that is definitely the culprit for a lot of glazes not fitting. I have found glazes that fit but the issue is if there’s any tiny section where the glaze doesn’t cover the inside, or pinholes, the whole thing is bust because the clay is nowhere near vitrified. From what I understand talc in high quantities at this temp is very refractory, so keeping it in that % i don’t imagine I could get it to mature, but this is something I’m still quite new to so I may be wrong. Just not sure what I might try to attempt to get it to vitrify 

[Babs]

Porous pots fine for plants, in fact recommended,  if not placed on your french polished grand piano or dining table, could just sit in virtrified saucer.....of course,  seepage  may change overall look of pot. If an inner saucered pot could fit in your beautiful pot then prob solved for the planters.