Clays are notoriously difficult to effectively preserve as they tend to pull the preservative in to itself, leaving the water stage under preserved.
We have found in our own tests, that Preservative K was effective when used at 1.5% but there are many other variable to take into account and you should only rely on the results of your own challenge tests.
Although some of the older publications still suggest using drops as a measure, this is not accurate as dropper sizes vary.
Do not blend different preservatives with each other unless you are experienced in this area and can have the result tested for efficacy.
If you are adding extra materials to a base product, you may need to add extra preservative as well.
Ensure the same preservative as the base product is used.