I mined sulfur crystals last week and I read the following on this guide:
To prepare the specimens [...] it is necessary to soak them in water to which ammonium hydroxide has been added. This neutralizes the acidic component of the matrix, and thus prevents the specimens from eating up any paper or cardboard that they might come in contact with. A small amount of sodium bifluoride is added in order to complex iron. Without this treatment, the matrix of many specimens would gradually turn a pale brown.
A quick Google search tells me that I can buy both products on the internet.
Does anyone know the concentration of ammonium hydroxide and sodium bifluoride necessary? Also, how long should one soak the specimen in the solution?