White pots are surprisingly difficult to achieve. I remember seeing a documentary about the colour which showed Wedgewood's research notebooks - it took him 144 attempts to formulate a recipe and method that he was happy with. The white glaze I currently use is one of about 30-35 I've tested, and generally speaking I'm happy with it. It even gives me a little romantic glow because it uses tin oxide. Tin mining was of course important to the history of Cornwall, home of my maternal grandfather, so it gives me a sense of connection.
However, all is not happy in the land if tin white. The tin has the unfortunate side effect of a tendency to 'crawl', pulling the glaze away to reveal areas of exposed clay below. Sometimes spots are small, like the rim of this jar.
Occasionally it is rather worse - as on this bowl.
Either way, every piece has to be touched up and then go in the queue to be put back in the kiln for a third time and re-fired. This generally solves the problem, but it's a bit frustrating to have work sitting on the shelf for a month waiting for the next firing - and of course it takes up valuable space in the kiln!
I'm currently making up a very large order for a stockist, 2/3 of which will be white - so you can imagine the dread of how much re-firing there will need to be.
Perhaps it's finally time to re-visit the recipe and see if I can alter it to stop the crawl, although in ceramics it seems nothing is ever straightforward... I could swap the tin for zirconium, but that apparently gives a slightly different colour and means you also have to adjust the silica content... or I could adjust the firing schedule and fire slower or hotter to see if that helps, but I can only test that with a full kiln and so every experimental change in the schedule risks a whole load of pots... Wish me luck!