100% ball clay projecting black dots after firing at 550 °C

[PeterH]

Which ball-clay are you using, and do you have a choice?

Organic Matter in Clays: Detailed Overview
https://digitalfire.com/article/organic+matter+in+clays%3A+detailed+overview

Note: According to F. Q. Al Khalissi e W. E: Worral (Trans. Brit. Ceram. Soc., 8,1982,pag.145) organic substances can be completely removed by treating the ground clay with water oxygenated at 30% vol. and heated for several hours at approx. 80 C.

https://www.potclays.co.uk/ball-clay-twva?search=ball clay

PS Have you checked for black-core?

PPS LOI looks like a simple way of checking the level of organics in your ball-clay.

PPPS Would pre-burnout of some of the ball-clay as a power help?
A thought inspired by this comment in https://digitalfire.com/material/ball+clay
Ball clay typically does not comprise more than 30% of a body recipe so its opportunity to burn away is sufficient.

[Bill Kielb]

Quote

“Thank you @Bill Kielb. But my chalk is for marking cloth.  It should be smooth like soap. The only problem is the dark spots.  I kept it on 600°C for 30 min today and please see the result below (..”

@bijoy

Actually since it’s only for marking fabric, do you even care about the impurities? Anyway, I can’t tell from your picture  if that’s better or worse or the same so if better I guess I would go for an hour hold and so on if better still.  At some point I think you need to figure out what it will take to fully remove the organics temperature and most likely the hold time as in hours and see if this still works for you.    The previous question of cleaner kaolin comes to mind as well.   BTW, my  mother always used bits of soap when she ran out of Taylor’s chalk. She would always gather the soap bars that became too small to wash with. This was fifty plus years ago though and the economy was essential in those times.

[Russ]

A talc would work. Most of the tiles produced by Daltile are talc based and pressed with tons of force into moulds. They stay together quite well and with little firing should remain carbon free and soft as soapstone which BTW is where talc comes from.

[Callie Beller Diesel]

Do you have an idea what the organics are? Some kind of plant material, perhaps? If they’re visible in the dry mix, it could just be you need to run it through a screen. If you’re making tailor’s chalk, it might be a better idea to find a gum or other organic binder that won’t stain fabric rather than trying to fire the mixture at all. Soap comes to mind as something that could be used readily, and would not affect garments. You wash them before first wear in any case.

[Callie Beller Diesel]

On 9/25/2021 at 7:49 AM, Bill Kielb said:

do you even care about the impurities

Carbon could indeed stain.