At the time, phenol came from coal as a by-product of coke oven gases or manufactured gas for gas lighting. Phenol could be nitrated to picric acid and converted to ammonium picrate, a shock resistant high explosive suitable for use in artillery shells. A telling of the phenol story is found in "The Aspirin Wars". Most phenol had been imported from Britain, but with war, Parliament blocked exports and diverted most to production of ammonium picrate. Britain also blockaded supplies from Germany.