"He who marches out of step hears another drum." ~ Ken Kesey
When I first started, with WB on flatstock, I would use glycerin as a retarder. Made manual printing in the hot summer months feasible.As far as thickening goes, evaporation is a possible solution. Other than that, unless I was trying to produce my own inks,I'd stick with what the mfgrs offer. Enough variables as is to throw some more in the mix. I'd be surprised if someone doesn'toffer a thickener.