Small spots on the shelf are likely from spitting, and can be caused by the boron. Large spots on the shelf and bare spots on the pots are different. When the glaze pulls away leaving bare clay we call that 'crawling'. It can happen when glazes are overlapped if the second dip is applied when the first dip is still too wet, or too dry. It can also be caused by the surface tension of the glaze during the firing, glaze thickness, firing parameters, etc. Floating blue can be prone to lots of different problems, including crawling.
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