That said-- I make a solid oil based salt scrub and do not use a preservative, but I state *explicitly* that you CANNOT introduce water to the scrub because it does not have a preservative. I state *explicitly* that a portion of the scrub should be removed for use and the scrub itself should not be kept in the shower.
I make a solid oil salt scrub, I add preserve to it simply because it will react with water and used in water. People tend not to follow directions, if they read them at all. The moisture in the air, steam from the shower period was enough for me to add a preserve since I know it has to come in contact with water. You should consider adding it simply because consumers just dont listen to directions....
For my bath salts, I sold them for years.. I added preserve simply because I added color to them, which was in liquid form. Although it dried, it was enough for me to consider a preserve for shelf life. I have tested with salts sitting in the shower unused, with them sitting in the bathroom on the vanity unused.. and the same ways opened...
The one without the preserve over time did start turning colors.... I am not saying put alot of preserve in it, but you should consider it.
Most of my products that I know will go into the bathroom... have a preserve...
The only thing that doesnt, is my lip balm I use to make..but it contains Vitamin E.
Now some say "Its not a preserve" some say "It Is"
Guess what... I tested it over trial and error and a lot of time over years.. and I have never had a problem with using VIt. E in my lip balm.
If you are unsure about preserves... its time to read more on them.