Mike is correct in that the sulfur precipitates over time but I have to put my two cents in and state that that sulfur never made a difference to me. For both B&W and color, I have simply filtered out the particles and nothing seemed to matter for the worse. Whenever you have liquid chemistry the opportunity always exists to mix partial quantities. Just be accurate with your measurements and it is best to use metric, even if the kit is quantified in avoirdupois. I had some RA4 blix concentrate (the black part, ferris-something) for a dozen years and after that time it kind of clumped to the bottom in one black mass. After much shaking it worked fine when mixed with the equally old ammonium thiosulfate part. I use potassium ferricyanide for that component now because I find it both easier and less messy. But, remember, that the mixture does not last more than about 30 minutes when mixed with the fix. - David Lyga