Sulfur sticks are used in industrial ammonia refrigeration systems as a minor-leak ammonia detector. A sulfur stick is made from a wick which contains particles of sulfur.
The sulfur stick is lit and smolders, something like the taper that is used to light fireworks. The color of the sulfur smoke is used to find the ammonia leak. When there is no ammonia, the smoke is colorless. When ammonia is present, the combined sulfur and ammonia vapors produce white smoke.
Steps:
Grasp the end of a sulfur stick opposite the wick.
Light the other end of the sulfur stick with a lighter or match. The wick should smolder like a fireworks punk and not maintain a flame.
Walk around the refrigeration system slowly and watch the sulfur stick. Pass the stick over and around the area where you suspect the ammonia leak. If the sulfur stick begins to emit white smoke, this indicates the presence of ammonia.