crack on INSIDE of bottom of pot

DKat,

Most common causes of bottom cracking are:

--excessive water left on the bottom while throwing,

--insufficient compression while throwing

--unequal drying, which causes stresses in the claybody (and may not be noticed until the expansion caused by the heat of firing.)

Cracks (can/will) usually show up on the inside for each of these reasons.

If your entire bottom is cracked through, I'd venture a guess that you dropped it

From what you described, I would think that it is a stress related crack. Have you done any testing, such as throwing the identical pieces, one after the other, using recycled clay for one, and new for the other? That should rule out the clay body if neither or both crack. If only the recycled cracks, I would check your recycling method.

If you have a "lump" where it cracked, and this is a bisque firing, take a hammer to the piece and see if there is an air pocket or evidence of bloating indicating over-firing. You might want to fire those pieces on their rims next time, to even out the heat, or place the piece on some wadding to keep the bottom off the shelf.

Hope that helps,

DKat, Have you swithced clay bodies? I have found that some clays are more prone to cracked bottoms. Standard #112 is one. I use it though because I love the effects I get with certain glazes. I haven't had near as many cracks with it since I started being extra careful to compress the bottom really well while making the piece and then also when trimming it. If it has no foot ring I still invert it and go over the bottom well with a hard rubber rib or a wooden rib. It seems to help to keep if from getting cracks. Sandi

112 is the clay body in question and these pieces did not have feet. How is it that you compress with trimming and how dry are they when you do this (I'm having a hard time imaging this since you are pressing down on air if you are pushing with a rib when the piece is upside down)?

I have always been careful about compressing with pieces I throw off the hump and I am always careful to compress 'down/backandforth' when opening any piece. I guess then that with this clay body need I compress 'in' on the bottom as I would with a piece off the hump? I think I can get into the habit of doing that. They were just such odd cracks (not S cracks, not in the center and on the inside of the pot rather than the outside), I dry slowly in a damp closet and I was throwing dry so I did not have a clue what the issue was. Thank you for the info and time (both posters). It is appreciated.

To compress the bottom while trimming a bowl...After I trim the footring and shape the sides of the bowl the way I want it, I take a wooden rib and go inside the footring on the bottom of the bowl and just go back and forth over the area inside the footring. If it is a very small area I use the rounded curved end of the wooden tool. You know the one that is indispensable for cutting away the excess clay at the bottom of the pot before running the cutoff wire under it. Your pot should be almost leather hard to do the compression on the bottom. The rib makes a nice smooth bottom. It almost looks burnished. Sandi

Thanks. I will without question do this from now on with 112. If a crack shows up after the bisque, I can easily deal with it but to think you have a sound piece, put in the glazing and firing to then lose it is somehow insult to injury.

Regarding 112 Standard... This appeared today on the Clayart listserve, and bears reading. I am copying it and pasting it here:

(Begin copied message) Hi to all 112 users, To continue my saga of 112 problems let me tell you the latest.

I did send clay samples to Ron Roy on my own without Standard sending them. Ron was gracious to test it for me and to take the time to call and discuss the findings with me. His dialometer tests showed a marked increase in cristobolite in the cone 6 body. This is not supposed to be there and causes cracking and shivering of glazes. I have had an abnormal amount of pieces splitting in two after several days of coming out of the glaze firing, some shivering of glaze off the edges of pieces and on any place the glaze may break of over relief and have had a lot of loss due to cracking of green ware at the bone dry stage (may be an unrelated problem, not sure) and sharp metallic looking growths growing out of the glazed fired ware ( definite fireclay problem). Any way Ron was worried about using this clay for dinnerware sets because of the chance of glaze shivering off and being ingested by customers. Boy did this open my eyes.

I shot off a letter to Standard and copied to Ron and to my clay supplier. I did not get a direct response from Standard but did through my supplier who was in contact with Standard. It is from this conversation that I learned Standard is sending new clay samples to Ron Roy for testing and they are addressing the issue. From what I have gathered there was a definite problem with the fire clay that they obtained from Christy Refractories that may be the culprit. Since I do not have first hand knowledge of this I cannot say for sure. Hopefully Ron will post his results to the list after his contact with Standard clay.

Ron, I hope I have not put you on the chopping block with this post but wanted those having the problem to know someone was following up on it. To all that I have had conversations and emails with having problems with the

112 clay, I hope this helps to know that you are not crazy and are not doing anything wrong with the way you make pots. (This was what I thought I was doing).

I have been reading up on fireclay in "Out of the Earth Into the Fire" by Mimi Obstler and see several references to excessive amounts of cristobalite causing glaze fit and fired strength problems. Of course it also states that minerals are highly vulnerable to change and should be closely monitored. It looks as though quality control practices leave a little to be desired if this problem as been continuing for a good while as noted by all the problems listed with this clay body. What puzzles me is I am sure the fireclay supplier must sell to more than just Standard Clay so why is the problem not showing up in more clay bodies?

I do want to make it clear that Julie at Standard Clay has been checking with me regularly to monitor my problem and had not just left me hanging. I do appreciate that fact and am sure that this is something that can be worked out.

My supplier has been ready and willing to assist me in solving this problem also and I would like to publicly commend Tammy with Mid South Ceramic Supply in Nashville. She always had time to discuss problems and offer help where needed. (END copied message)

Thought that you wold be interested. It may not be anything you are doing or NOT doing. It may just be the clay!

"wayneinkeywest" wrote in news:nYXxc.1802$ snipped-for-privacy@bignews4.bellsouth.net:

There is only one reason why clay shivers and falls off a body.

The glaze has not been adjusted to fit the clay body. That is the body shrinks more than the glaze does and tension is set up in the glaze and it falls off.

All you need to do is to adjust the glaze shrinkage by manipulating the glaze formula.I suggest that you go to the Insite web pages. They have a great number or articles and this problem is well covered there. If you don't know how to do this google Insite

There will be nothing wrong with the fire clay, only that an adjustment hasn't been made to the glaze formula to take into account what the formula of the fire clay is. All clay is silica and alumina in different proportions and what it is called or retailed as is immaterial.

All you need is a basic understanding of glaze chemistry.

If you like post up your glaze recipe and I'll run it through Matrix for you

Regards

John W

John: I'm simply passing the information on for the cracking issue, not glaze shivering. I do not use 112, personally. Just thought it might be an issue with the clay, and not a compression problem, so when that info presented itself...

Thanks, though for offering to run it through Matrix. I use GlazeMaster, myself :>)

Best Regards,

Wayne Seidl Key West, Florida, USA North America, Terra Latitude 81.45W, Longitude 24.33N Elevation 3.1 feet (1m)

I just now found this post (my server at home only holds things for one day). We actually had someone in the studio who had chunks of their pot just drop off. Again, these are things happening that none of use have seen before and they match what you have posted. Thank you very much for sharing.