Mustard is made by adding liquids to mustard seeds or their powder, which activates the chemical reaction in them to make them spicy. How spicy a mustard becomes depends on the temperature and acidity of the liquids, and on how long the reaction is allowed to go on - heating or cooling the mustard significantly will stop it, keeping a mustard cool once it reaches the desired hotness will preserve it at that spiciness level. As it is, I wonder what the ratio of solids to liquid was and if it was a pure mustard (without other spices or additives) since I have never even heard of a semi-solid mustard...