One of the pleasures of wood/soda firing is the use of flashing or effect slips over your clay or clays. I mainly use two clay mixes, one higher in iron than the other – so I need effective slips for both clays. The flashing slips for the low iron body have some iron in them, either from my clay or a ball clay, whereas the flashing slips for my high iron clay mix have only china clay and often zirconium silicate added.
The other factor that influences the slips is in their application, which is mainly by pouring the slip over the various forms at the leather-hard stage. So shrinkage mis-match will cause flaking, which can be a formulation, thickness or application timing issue – and each of my clay mixes has different shrinkage from wet to leather.
Additions like fire clay, zirconium silicate, alumina hydrate and feldspars can also make big differences in the final results. At the moment my focus is on a new slip for my darker clay and the two pots below both have the same slip on them and applied in the same manner – the bottle is the dark clay mix, the caddie the light clay mix, both in the same firing – but not necessarily exposed to the same amount of soda.
I end up having to test these slip experiments on actual pots as the effects can vary in different places in the kiln – so a small test tile doesn’t work. Which also means it takes a number of firings and small tweaks to get a result I’m happy with.