Making so many forms has used a lot of glaze, so a need to make some more. I adjusted my purple glaze to contain a slightly larger proportion of cobalt and the test tiles (11 and 11a) looked fine, so I made a pot. I really dont know what I did with this one (6/11a), though it must have been a weighing mistake. It was fired at the top of the trolley kiln at cone 7. It is clearly very underfired, so went in again. The cone 8 was well over (so a little above temperature) but the surface, though interesting, is still a little too matt. It is shown below. It was not my only glazing problem. I had tested the tin containing glazes over those rebisqued with a chrome containing one and had thought that the combination did not create a risk of chrome tin pink. How wrong I was. refired again at cone 8 the purple glaze (11a) is attractive if still a little matt. The yellow blushed with pink looks good with it -though this is not a combination I will be attempting to reproduce! Unfortunately these are not really in keeping with my colour range so I will have to try again with purple. 11b is on its way..... I am still having problems with crawling. This is despite using a fine slip at greenware stage, scrubbing each piece with soapy water prior to glazing then applying a thin layer of glaze followed by two or three thick layers. In deeper grooves or dots the glaze can trap a bubble which is difficult to shift; using thinner glaze initially definitely helps with this but it sometimes requires some mechanical disruption. Unfortunately this does not explain crawling on flat(ish) surfaces. This is an extreme example with the glaze most prone to crawling (teal), applied to a large convex base plate. The piece was fired at cone 7. The glaze has pulled away from a surface crack even where the application is thinner but there are no cracks or gaps at the centre of the large bare areas at the apex. It has been possible to rescue some of these pieces by heating up and then adding a thick dab of glaze in the gaps. The white areas on the picture show the beginning of this process. I have been please to see that the surface cracks did not open up more and have been at least partially covered. Re-firing does add extra 'heat work' to the glaze which becomes glossier and more translucent. I have tried various ways of brushing the glaze on to reduce the incidence of crawling. Physically attacking any hint of bubbles seems to help. Thinner glaze applications also do. Some times the glaze appears to 'bubble' as it goes on, though the liquid in the pot seems smooth. Is it the brush? I have not tried changing it but that would not explain why test tiles have been largely free of the problem and the largest pieces most prone. It must be my application. Retouching with a thick dab of the same glaze is not quite straightforward. Some of the glazes have been settling really quickly, so more oxide or stain is at the bottom of the pot. There is then a risk that the repaired area is a different colour. Glaze settling is a problem which is time consuming and risks uneven colour even during glazing of a single piece. I needed to try changing the base glaze. Following advice, I had previously tried adding 5% bentonite and found that the glaze clumped as is went through the sieve. I needed to look again
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AuthorI am indulging my passion for ceramics by undertaking studies for an MA at UCLAN Archives
August 2021
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