SweetheartSister got a reaction from Rae Reich in Overglaze Durability Question
I'm still new-ish to ceramics and was wondering if someone could answer this question for me.
In the past I had problems with transparent overglaze crazing/pinging on earthenware and it has made me quite paranoid, especially as I sell my pieces. I switched to porcelain many months ago because I was told that its composition makes it less likely to be prone to crazing (as it can vitrify properly whereas earthenware always remains a bit porous?)
Anyway, to cut a long story short, with all porcelain pieces I've made so far I've done a test of leaving them in the freezer overnight, removing them, putting them in the sink and immediately pouring freshly boiled water over them. I've done this many times on many pieces and there are no cracks or signs of crazing to be seen - they remain in perfect condition.
My question is, does this mean I can feel confident that the pieces will last for many years to come if people take care of them? The pieces I make are jewellery and are small.
Any advice is much appreciated - I'm just looking for some reassurance that the shock tests mean the glaze has adhered well and that there isn't a problem with the glaze fit or coefficient of expansion. My biggest fear is that a customer will come back in a few years complaining about craze line imperfections.
I think someone said to me on this forum a while ago that the freeze/boil test was the best way to see if the glaze was going to last or not.
SweetheartSister got a reaction from Chilly in 1st Porcelain Glaze Firing
Thank you for this very detailed explanation, Min. I honestly appreciate it so much. I feel a bit stupid, firing the underglaze to vitrification temperature - I know that glazes don't take well on vitrified clay so I'm not sure why that completely went over my head. With regards to these pendants, they actually look really nice as they are without an overglaze, so for this batch I will leave them as is:
20190923_074045 by Rachel Brown, on Flickr
If I wanted to repeat this process to accomodate a transparent overglaze in the future, do you think the following would work:
1. Bisque fire casts to Cone 05
2. Paint on underglaze and fire to cone 015 to burn off impurities
3. Paint on overglaze and fire to cone 7 (will mature at 6) to create final, vitrified piece.
I don't think I could do liambesaw's suggestion of painting onto greenware, as the pieces are so fragile before bisque firing - it is very easy for example to chip off an ear on a pendant once dried.
With regards to crazing, the supplier has recommended this exact overglaze to pair with the slip I am using from the same supplier. It is designed specifically for porcelain. So hopefully it will be okay, so long as I fire to the correct temperatures.