The wet plate process relies on an acid silver bath and acid developers to avoid fogging on plates. For this process, alkaline chemistry is bad. Test your chemistry periodically to insure that it's happy and healthy. Litmus paper provides a quick and easy test to see if your chemistry is either acid or alkaline so you can then take appropriate corrective action.
The red litmus paper will change to a blue color in the presence of alkalinity, and indicates a pH in the range of 6.8 to 10.
The blue litmus paper will change to an amber color when exposed to acidity and indicates a pH in the range of 4.3 to 6.8.