Is it possible to glaze fire chemically sealed clay?

[gawann]

Hi everyone,

does anyone know what will happen if I fire some vases that have been glazed on the outside but sealed with chemical sealant on the inside after the bisque firing? The glaze fire in the studio where I burn my cheramics burns on too low a temperature (1130 degrees celcius) to seal the clay I use, therefore I wanted to try with a sealant. Unfortunately the description on the packaging was somewhat misguiding and I as a ceramics novice realized a bit too late that it obviously only make sense to seal after the final firing..

If the only risk is that my pieces burn or break, that would be okay (they are useless as they are now anyway), but if there is a chance that they will intoxicate the air around the oven or implode and/or damage the other things in the oven, then I wouldn't want to dare that..

The sealant is called "tondicht" and contains 2-Octyl-2H-isohiazol-2-on, 2-Benzisothiazol-2(2H)-on, 5-Chlor-2-methyl-2H-isothiazol-3-on, and 2-Methyl-2H-isothiazol-2-on.

Would be grateful for any answer :)

[Bill Kielb]

1130  degrees C is roughly cone 4/5. Just curious what cone does your clay mature at and what cone does your outside glaze mature at. Seems like you should be using a cone 5/6 clay and 5/6 glaze. Now for removing the sealant, it will likely burn off in a bisque firing but very likely will fume and maybe even stain so if their kilns are well ventilated you may be able to remove it by bisque firing. I am not sure anyone here will be able to say with 100% certainty that nothing will happen. Is there any English literature available on you sealant?

[gawann]

Thanks a lot for you answer, I really appreciate it!

Yes, my clay needs 1250 degrees C or so to mature, but since we started off using glazes that were meant for lower temperatures, we didn't fire the clay on that temperature (I only started learning about the maturing of clay very recently and never paid attention to that before). The things we have made so far might not be perfectly water-resistant, but after a glaze firing at 1130 degrees, cups, plates and other things for own use are mostly fine and even go in the dishwasher. But will definitely consider either changing to a clay that matures at a lower cone or fire the pottery somewhere else on a higher cone and buy some higher-cone glazes in the future.

There isn't much information on the sealant, unfortunately. I called the producer today and they also never tried firing clay to which this sealant has been applied. For smaller pieces, I managed to remove the sealant by polishing it off, but for the vases I will probably just bring them to the studio and see if they're willing to give it a try or not.

Would indeed be a good thing if firing could remove the sealant. Do you assume a glaze fire at 1130 degrees would have the same effects, or would it be smarter to do another bisque fire first to see if the sealant has been removed and then go on with the glaze firing?

[Bill Kielb]

@gawann maybe as above but to add you probably could test this on something inert as well  and carefully torch it off a cookie or test tile outdoors to see what it will do. Is it flammable, does it smoke excessively. A propane style torch will probably only get 600- 800 C so if it works, then a bisque fire likely will work just fine. You will also have an idea if any smoke will be generated. I think likely everything will work out fine actually, not many things survive kiln temps.

For your test, use safety glasses, respirator, heat evenly slow and cautious  outdoors.

[liambesaw]

It's gonna be stinky as hell with all of those sulfur based aromatics (anything with thio or thia).  But should be fine to bisque again.

[Bill Kielb]

2 hours ago, liambesaw said:

It's gonna be stinky as hell wit

Not sure, it seems like mostly preservative and suspending agent actually when you look up various components,  maybe use water as a carrier.  The chlorine component strikes me as a concern. Plenty of ventilation for the test is definitely in order.  The other thought is it may wash off reasonably with soap, water and some brushing, reducing the amount that needs to be fired away.......... if it does smell awful.

[liambesaw]

Dude thiols and thials are some of the stinkiest compounds known to man, I was experimenting using thiols instead of mercaptans (which are stinky enough on their own) for lustres, and had the local gas utility cruising the neighborhood looking for leaks it was so strong.  And we are talking about total volumes of 50ml.

Since the sealant above appears to be mostly aromatic petroleum byproducts it shouldn't be too bad, most of that will evaporate within a few days, so I wouldn't expect there to be a fire or too much chlorine left over.  But may still be pretty stank.

[Bill Kielb]

Hmm, Maybe wash it off thoroughly first is in order.

[oldlady]

looks like a lesson well learned so throw them out and start again doing everything correctly.   what instruction have you had in any form?

[gawann]

17 hours ago, Rockhopper said:

I would definitely recommend burning off the sealant before applying glaze.  1) You'll likely have a hard time applying the glaze to the sealed surface.  2) If you apply glaze over the sealant, the sealant will likely bubble up through the glaze as it burns out - probably leaving pin-holes (or bigger) in the glaze.  It could also cause large areas of glaze to flake off, or slide downward.

Do you have a test tile, or other small un-glazed piece you could apply the sealant to, then bisque fire ?  That would let you find out what's going to happen, but with smaller amounts of "combustion products" (smoke & fumes) as it burns.

I'm not planning on glazing any of the sealed surfaces, but I already applied glaze to these objects on on one side (and sealant on the inside). 

I made two small soap dishes to which I also applied sealant on one side, and I actually managed to remove almost all sealant from one of them by polishing with sandpaper. Think I'm taking this one to the glaze fire. As for the other soap dish, I could concider using this as a test.. Would be interesting to know what would happen.. If I only had my own kiln there would have been less things to worry about. 

[gawann]

9 hours ago, Bill Kielb said:

Not sure, it seems like mostly preservative and suspending agent actually when you look up various components,  maybe use water as a carrier.  The chlorine component strikes me as a concern. Plenty of ventilation for the test is definitely in order.  The other thought is it may wash off reasonably with soap, water and some brushing, reducing the amount that needs to be fired away.......... if it does smell awful.

Your point on removing as much as possible of the sealant first definitively seems to make sense. I was surprised to find out that the sealant was rather easy to brush off with sandpaper, that is it doesn't go that deep into the clay as I expected. A few of the pieces have such a small opening that brushing etc. on the inside is impossible though. It's really no tragedy if I have to throw them away (or remove the glaze and apply sealant to the outside as well, and skip the glaze fire), but your answers are really helpful!