...so how is one supposed to properly supplement it for various starting waters without knowing the percentages? I've seen recipes that simply state to add X amount of BWS, but these are generally recipes based on tap water - I always use spring water with known ppg values (which would certainly need adjustments to match good tap water, let alone tap water with BWS added).
Is the case simply that experienced brewers who care about their water just don't use the pre-blended Burton Water Salts? If that's the case, is the proper way to replicate a recipe calling for BWS to always adjust to match Burton UK water, regardless of style?
Basically. There are different blends of Burton water salts (I know of at least 3- one has gypsum+epsom, one gypsum+epsom+calcium chloride, the last gypsum+paipan). The pre-mixes are generally regarded as a crapshoot. That said, some people use them sucessfully.
When I make an ESB, I measure out my own water salt additions. Following the advice in "Designing Great Beers", I usually shoot for ~250ppm Sulfates (Burton-on-Trent is something crazy... 600+), Calcium ~100ppm, Magnesium about 25ppm.
However, I haven't done a strict comparison yet, and there may be better numbers to shoot for.
For anyone else stumbling across this thread, I found this:
From the back of a packet of 1oz. BWS by Brewcraft of Portland,OR, USA:
Contains Calcium Sulfate, Potassium Chloride and Magnesium Sulfate. Used to duplicate the famous Burton-on-Trent brewing water, known for it's [sic] pale ales. Use 1 tsp. per gal. to increase hardness of water by 889ppm. Add prior to boil.
Breakdown of the water hardness: 135ppm Ca++, 388ppm So4--, 19pp Mg++, 183ppm K+ and 165ppm Cl-.
Reorder Code: 990-A-1