I believe that what I think they call a curl test is what they use to determine if something is tanned or not. I believe that the hide is heated up to a certain temp. and how it reacts lets you know. I never really paid much attention to how to go about doing that, because I was satisfied with the end result of what I was producing, so I just accepted that it was preserved either by turning into leather or some other chemical means that I felt was suitable to the outcome I was looking for. Tanning is a form of preservation as well.