watch-balln. a ball of clay used to test the temperature of a glaze kiln (see sense 22 above).
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society > occupation and work > equipment > furnace or kiln > [noun] > part of
watch1606
fire door1765
fire bag1796
watch-ball1839
ring wall1863
1839 A. UreDict. Arts 1016 The watch-balls of these first rounds have generally not so deep a colour as if they were tried in a furnace three or four months old.