A feldspar is typically referred to as 'potash' if there is significantly more potassium than sodium (typically there will be 2-5% Na2O). If the amounts are closer to equal they are termed potash-soda feldspars (or vice versa). 

The main vitrifying agent in ceramic body is feldspar. The majority of white ware bodies contains good proportions of feldspar. It acts as flux in the ceramic industry, the flux is defined as that portion of the body which develops glass phase and this is provided by fledspar. The amount of flux in a ceramic body should be only in such a proporation as to develop the desired amount od vitrification. if excess of flux is added, the fired body becomes very glassy and consequently brittle.
Potassium feldspar melts at around 1200C (higher than soda feldspar) producing a more a viscous melt than does sodium feldspar. It can also produce a crystalline phase (leucite) that contributes to opacity. Thus, in industry, potash feldspars are often used more for matte glazes while soda feldspar is used for glossy glazes.