The H that was brought in to the south west of the UK was always in rock form when it reached the user, it was H in the base form with other cuts averaging 30% purity.
The lump was always placed in the cooker and vitc or citric acid added with a small portion of water, heat and stirring led to the product dissolving into solution, yes it may have been better practice to use less heat and smaller amounts of acid, but many addicts think that if anything remains in the cooker they are loosing product.
There are video's on youtube stating that if an excess of acid is used with to much heat the H is partially broken down into morphine and monoacetylmorphine, and it is ok to leave small deposits in the cooker.
The heating process is used by addicts in an attempt to sterilize the shot and get everything to disolve..
best practice is to use a very small amount of acid and minimal heat, do the shot and then if deposits are left maybe hit it again with more water and tiny more acid, thus avoiding breaking down to much H.