I got a quick question. Just picked some fresh strawberries and was looking at an old church cookbook. Was looking at maybe making some strawberry jam. I use to help the neighbor make it back some 20 years ago. While looking through the book at one recipe the wife asked me what Alum was. I can't recall ever using it and don't remember what it is.....LOL. What is this stuff? I was thinking it might be used for preserving. Thanks for any info.