frustrated

[PatJ]

Hi everyone,

 

 

Thanks again for all the tips that you have sent me on some earlier messages. I have a new question. It seems kind of freakish to me. Last night I was slipping 2 of my pots (I do hand coiling and don’t use a wheel). 1 bowl and 1 cooking pot, both micaceous clay bodies which I have dug from out in the woods.

 

 

Let me tell you what happened. So I was working on the bowl; I slipped the outside 4 times with a non micaceous slip and then stone polished it to a real nice sheen. Then I slipped the inside 3 times with micaceous slip. I then began softly stone polishing this, not so much for a shine but just softly to smooth out the texture. I rubbed it a few times and then bam! It cracked into 3 huge sections and is only barely holding together.

 

 

So then I went to work on my cooking pot. I slipped the outside 4 times (I usually wait about 15 minutes before applying a second coat; the micaceous slip dries fast) and buffed it with a cloth. Then I slipped the inside 3 times and then smoothed out the texture by lightly polishing with a stone. Once this was done I applied some vegetable cooking oil in very small amounts to the exterior surface and then began polishing with a stone (I do this with most of my pots). Next thing I know I have 4 cracks that just appear, one of which runs nearly a 3^rd of the way around the body of the pot. All of the cracks are on the external surface and don’t appear to have transferred to the internal surface.

 

 

I have been working with micaceous clay for a year and this has never happened before. This is a very strong clay body and once the pots have managed to pass the drying stage, they are generally in the clear and slipping and polishing are never a critical thing to the structure of the pots. I didn’t do anything different then what I normally do

 

 

Sorry this message is long. The only thing that I can think of that was different is that we are in the monsoons right now and it was actually raining the night I was doing all this (and it was cool and humid). New Mexico (where i am at) is very dry with VERY low humidity but that night it had been pouring all day. Did this have something to do with it? Any thoughts?

 

 

The cooking pot is no longer saleable, but is it still usable for cooking? Or is it a total loss too?

 

 

 

[Chris Campbell]

Here is what I think happened but others feel free to jump in.

I am assuming your bowl had dried before you tried to do this slip and buff technique.

 

You slipped the outside several times, so it was much wetter than the inside, creating stress between the inner and outer surface. I think if you had stopped then and let it even out for a few hours or even overnight you could then have done the inside with no harm. Sometimes you get away with it and sometimes you don't.

 

In the future try doing all the insides one day and all the outsides the next.

 

I would not use the bowls at all since this has weakened them ... I would not cook with them and risk the pot collapsing with hot food inside.

Put em in the recycle bin.

[PatJ]

Here is what I think happened but others feel free to jump in.

I am assuming your bowl had dried before you tried to do this slip and buff technique.

 

You slipped the outside several times, so it was much wetter than the inside, creating stress between the inner and outer surface. I think if you had stopped then and let it even out for a few hours or even overnight you could then have done the inside with no harm. Sometimes you get away with it and sometimes you don't.

 

In the future try doing all the insides one day and all the outsides the next.

 

I would not use the bowls at all since this has weakened them ... I would not cook with them and risk the pot collapsing with hot food inside.

Put em in the recycle bin.

 

 

 

Hi Chris,

 

Yes, I will be recycling them. They were bone dry and had been sanded and were sitting on my shelf waiting to be slipped for almost 2 weeks. I'm just flabergasted because I have been doing the same routine for over a year; slipping and polishing, inside and out, all in a single evening and then warming them in an oven the following night to prep them for firing and this has never failed me. I usually wait about 15 or 20 minutes between adding slip coats and I almost always add oil at the end to give it a little extra shine.

 

I have another cooking pot that is almost dry. I will slow things down a bit. Slip only the exterior one night, the interior another, and then maybe oil it a third night. Maybe this will work. Anyways thanks for your comments. Very much appreciated.

 

Patj

[Guest HerbNorris]

Could you slip them when they are almost leather hard, since you are going to sand them anyway? That might help eliminate some cracking.

[azjoe]

My guess is along the same lines as Chris'... a drying problem.

 

Drying and re-wetting clay is a touchy business. I've always believed that "micro-cracks" form when clay is lightly stressed when it's almost dry. Micro-crack is my way of saying "tiny unseen cracks and a general weakening of the clay wall". A ceramics engineer would probably use different words... including something about the breaking of the electrical bonds that bind the clay platelets together as they dry. That's a little to techie for me. We all know uneven drying and physically manipulating clay when it's too dry produces stresses that can result in warping, cracks or breakage. When the stresses are more subtle the clay platelet bonds are broken in a much smaller (and often internal) regions of the vessel wall... hence my term "micro-crack". In my mind there is always some "micro-cracking" and so I'm not overly anal about it... the piece just needs to hang together until the forces of firing come into play to heal some of these ills.

 

When you slipped the outside of your pot the outer surface of the dry pot wall absorbed some moisture from the slip. When dry clay absorbs moisture, it expands proportional to the amount of water added. Since it was the outside surface, it could expand as much as needed. This expansion created some internal stresses deeper in the dry pot wall. Many factors... clay, wall thickness, amount of water, etc. determine whether these stresses crack the clay. In your case it didn't crack, but some micro-cracks likely occurred at some depth within the wall, weakening it in various regions.

 

When you then slipped the inside surface, a similar thing occurred. However, assuming the pot was round (or spherical), the expansion of the inside surface was constrained by the dry outer portion of the pot wall. Depending on the expansion factor of the clay, how much water was in the slip, etc. the outer surface of the wall experienced considerable tension stress (ie, pulling apart... much like blowing a balloon up inside the pot would cause). Most likely, additional micro-cracks formed in the wall, weakening it even more. The additional stresses of handling/burnishing then caused a crack.

 

You're concerned why this time (and not previously) so you can avoid this in the future. Any number of reasons... differences in the clay (clay can vary from spot to spot), difference in the wall thickness, slip water content, etc. Most likely it was the rain/humidity that was your downfall. Slip coats that dried previously in 15-20 minutes didn't this time... so the clay absorbed more water with each coat, hence the clay expansion was greater than before. The dry portion of the clay wall wasn't strong enough to withstand the additional expansion and more micro-cracks formed than before.

 

In my area (the Arizona desert) the humidity a large portion of the year is under 15%, sometimes as low as 4-5%. Under these conditions moist clay, slips, and glazes dry quickly... and unevenly if one is not careful. Even in our dry climate, I would not consider a layer of slip or glaze to be bone dry in 15 minutes, however. Vessels I throw in the morning can often be trimmed late in the day... if I intend to leave them until the next day they must be covering well or they're way too dry. However, during the summer months (what we call the monsoon season) the humidity is frequently much higher (35-40%) and when it's rained it can get over 70% (sometimes only for hours, sometimes a day or more). Accommodations must be made as things dry much differently across these varying conditions

[PatJ]

Thanks. This explanation was very detailed and very insightful. It has also made me worry about some of my other practices.

 

I was taught that when working with micaceous clay, you let it become bone-dry, then sand it, then slip it. With the exception of this last experience, this system has worked for me. However, the thought of leaving internal micro-cracking is a bit unsettling. I would like to get away as much as possible from any practices that could cause excessive micro-cracking.

 

 

I like the idea of slipping the pot when it is leather hard, but then that leaves the questions of how do I sand the pot then? Sanding after slipping would sand away the slip before it ever erased the imperfections on the paste surface, correct? This is an issue I have with a non-micaceous clay that I am also experimenting with. Any attempt to slip it after it is dry seems to crack it so I do it when it is leather-hard which works, but then I don’t want to sand it because I am afraid to grind off the slip. I have not perfected my skill to the point that I think I can get away from sanding, but I am working on perfecting my skill.

 

 

Also another issue is when I do sand this non-micaceous clay it smoothes out the clay surface, but reveals lots of bumps (exposed temper) which is another reason why I am tempted not to sand this type of clay. Do you have any thoughts on this?

 

 

One last question; when slipping a spherical pot, would it be better as a means of reducing micro-cracking so simply reduce the number of times I slip the inside?

 

 

Very much appreciated

 

 

Patj

 

 

[Chris Campbell]

You got me very interested in this topic and while surfing the net I came upon Brian Grossnickle's site and watched his videos of the process.

 

http://www.micaceouscookware.com/forum/

 

His pots appear to be bone dry when he hand applies and scrapes off the thick slip. Note that his 'slip' is more like thick clay so there is not that much water in it.

They also appear to be bone dry when he sands so I have to believe he waits between these steps.

They appear to be bone dry when he applies what looks like terra sig and polishes. This explains the smooth surface as terra sig will fill and hide minor blemishes created by sanding. Terra sig also creates the lovely silky surfaces in primitive wood firings.

 

So I would say slow down ... Slow while you are forming so all surfaces are equally thick ... Slow down when wetting the surface and make your slip this very thick mixture to cut down on water absorption ... Make sure the piece is bone dry before you move to the next step. It is easy to gauge bone dry ... The pot should not feel at all cool.

 

Looks like a lot of fun!!