Overglaze Durability Question

[SweetheartSister]

Hi everyone,

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.

[Bill Kielb]

1 hour ago, SweetheartSister said:

Hi everyone,

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.

The tests likely indicate this will be a reasonable fit for many years. We do not have a real test that indicates specifically how long, there is no such predictive test available that is a reliable predictor. There are some anecdotal test, basically what you have done. In my opinion What you have done is very reasonable and a good indication this match will likely last a very long time, especially for jewelry.

[Min]

If you really want to make sure the glazes won't craze or shiver then I'ld do a couple other tests. First off, stress test them by heating glazed samples in the oven to 300F for 20 minutes then plunge them into cold water. If not visible crazing then increase oven temp to 310F and repeat. Increase oven temp to 320F and repeat a third time. Rub sumi / calligraphy ink on the pieces to check for crazing. The temperature difference is greater with this test than the freezer / boiling water test so more likely to show crazing from the increased stress. 

One other thing that can cause delayed crazing is if the clay takes in water over time, this isn't likely to happen with jewellery but again if you want to triple check you could measure the absorption of the clay. 

To check for shivering then make up some thin walled cylinders and heavily glaze the inside only. After firing put them in the coldest part of your freezer overnight then into the sink and fill with boiling water. If the expansion of the glaze is too low either the pot will split or the glaze will shiver off. This test probably isn't necessary for jewellery but if you want to be super cautious you could do it.