Miletus McKee- Talc and slipcasting

[Miletus McKee]

I recently learned that talc has been  removed from clay for fear of it causing cancer.  I was  pouring molds this summer and found that many of the large pieces we poured were collapsing before they were dry enough to take out of the mold, and a friend is having similar problems with her pieces collapsing and cracking.  Does this problem have anything to do with the absence of talc in the clay, and if so, is there anything that can be done to remedy it?

[Callie Beller Diesel]

Hi and welcome! I’m going to edit your post title so it gets more information, and move it to the slipcasating section so it gets better variability. 

I don’t know if the casting slip you’re using has had talc removed from it entirely, but we’d need to know the brand to help you dig for information. Talc as a mineral is often, but not always found with natural levels of asbestos. The industry standard source of asbestos free talc used in ceramics became unavailable a couple of years ago, when Daltile decided to stop selling it to the rest of us. There was a big scramble to find replacements, and while most pottery clays and glazes have been reformulated with alternatives, I can’t talk about the state of affairs with casting slip producers. We’ve got a pinned thread describing the drama here, if you want the background. 

I think there’s other reasons the casts could be collapsing, but I’ll let others speak on that. 

[Hulk]

The mid-range clays that IMCO* offers had been reformulated after C-98 ("Texas talc") became unavailable, however, I'm not certain what they are using instead.
The difference I'm seeing is in some of my glazes - the color is a bit different.

Next time I'm there, I'll inquire.
My guess is they are using another talc - they offer a Cimbar talc under their materials listings...
 

*nearby clay, material, tools, etc. supplier; they carry Laguna and their own line of clays.

[Miletus McKee]

The slip I used this summer was Laguna, and my friend uses Macky

[Mark C.]

It was Nital or HT 100 talc that had asbestos in it not the other talcs

Only that on talc out of many kinds has an issue

My guess is  Slip has talc in them depending on firing temps.Same deal with clay. Low fire clays ofen have lots of talc in them and they still do.

PS that talc was taken off the market many years ago now.

Edited October 10 by Mark C.

[Jeff Longtin]

In my experience cast pieces don't collapse due to the clay body formula, instead they collapse due to the process.

Are you casting pieces that are large, and bulbous, but have a small sprue hole? If you do, and your slip is thick, you may be creating a vacuum that causes the piece to collapse, inside the mold, when you pour out the slip.   

Are the pieces collapsing when you use reclaimed slip? I find reclaimed slip to be terribly flimsy.  (If used at 100%.) Thats why I always add reclaimed slip to new slip to reduce this problem. 75 new -25 old is a good ratio. 

One other possibility is that you're using a clay body that is over deflocculated. Are you adding deflocculant to the slip? If you're buying the slip in Laguna containers it probably comes direct from Laguna. If you're buying it in larger quantities, 5 gallon buckets for instance, and your supplier is mixing from dry, THEY may be over deflocculating the slip. (A possible way to detect this is to monitor the amount of water you have to add to make the slip pourable. If you do so, day after day, that may indicate a deflocculation issue.)

 

[PeterH]

Perhaps some photos if the problem is continuing?

[Chilly]

On 10/10/2024 at 1:19 PM, Jeff Longtin said:

large, and bulbous, but have a small sprue hole?

@Miletus McKee if this is the problem, put a drinking straw into the sprue hole while you pour - it helps air get in quickly, thus preventing collapse.