Sulfur is available from agricultural supply stores where it is sold as a fungicide under the name 'dusting sulfur'. It is a fine powder mixed with a few percent of calcium carbonate. The calcium carbonate may disturb delicate color compositions, but for most purposes dusting sulfur works well. If a purer form of sulfur is required, sulfur may also be obtained from drug stores sometimes. However, these often sell 'flowers of sulfur', which has been purified by sublimation and which contains some acid. This needs to be neutralized before use as it could cause spontaneous ignition. To do this, allow 100g of this sulfur to soak in a liter of water/household ammonia (1:5). Stir well occasionally and measure the pH. It should still be alkaline after two days, after which time the sulfur may be filtered and washed with hot water to remove the ammonia. Check the pH of the washing water while filtering. After it has become neutral, flush the water away with ethanol and allow the sulfur to dry. Mix the dry powder with 2% magnesium carbonate to neutralize any acid that may be formed in reactions with the atmosphere.