Contact vaporization is the more old fashioned way of creating herbal vapours for nutrition. We’d not suggest it now, but it is worth carrying out an analysis – for no other reason than to see what not to do!
To begin with take a look at this diagram:
The whole intention is to create sublimation without any overheating. The problem is that in a high pressure environment this can be difficult. Various methods are sued to reduced the air pressure within a system – from the fans of the volcano vaporizer to the airflow created as the user draws through the whip on a model such as the silver surfer vaporizer – these all reduce the need for contact vaporization and increase the chances of it occurring by another process such as convection – which has a much gentler and even heating.
With contact vaporization, then no matter how small you think you have ground up the herbs it is impossible to get them all to reach temperature at exactly the same moment – gently rolling the bowl might help a bit. On an industrial scale then a full rotating drum is used.