Primer Pulling Away from Shelf

[Ernsglass]

I am using a Hotbox 8 and Hifire kiln wash. I apply several coats of the wash, fire at 450* for first firing. Once completed, and shelf sits a day or two, then I fire small piece of 90 coe dichro capped with clear. Firing goes well. HERE IS THE KICKER. Next firing, same glass, same firing temp 1450*, firing goes well, BUT when I check the glass after cooling, it pulls the shelf primer off the shelf on the back of the glass. Can someone add some thoughts to what I may be doing wrong, or need to adjust. Will be glad to answer questions if you need more information.
thanks
Ernie

[rosanna gusler]

Put a fresh coat or two of wash between firings. No need to prefire kiln wash.

[lmartine]

Thanks for the tip Rosanna!

That begs for the next obvious question: if you keep adding 1-2 coats before every firing, when do you now it is time to scrape it all off?

Lucie

[Valerie Adams]

I routinely get 4-5 firings before scraping and re-coating a shelf. I re-coat when there are chips or dings that I wouldn't want my glass to pick up.

I seldom fire above 1440° and almost always have to re-coat after firing opal glass, since it tends to stick to kiln wash more than transparents.

I have several prepped shelves on hand so I'm never without a fresh one ready to go.

[Stephen Richard]

Whenever you go above a low fire polish temperature ther is a chemical change in the kaolin that forms part to the kiln wash. This means it sticks to the glass in the next firing.
Yes, you can cover the old kiln wash up after every firing. You will reach a limit where lumps begin to come off. If you are lucky, no problem will be caused to that last piece. It is time to scrape all the kiln wash off and start again.
However, it seems best to me to remove and recoat each time. Four coats only are necessary - one up and down, one across, Antwon on the diagonals. Takes only a minute or two. Same for removal - only coarse sandpaper and light rubbing for a couple of minutes.
Seems a small amount of time in relation to the risk of stuck kiln wash. And if you have two shelves, you will have one drying while the other is firing, not that a damp shelf is a problem on anything except large shelves

[rosanna gusler]

I make my own wash. 8/2 alumina/ kaolin plus some cmc for green strength. Super easy to scrape off. I use 1/1 for molds.