keep track of material

[Michael Stole]

Thanks to all ppls replying to my previous posts.
Here is another potential problem I really can not come up with a solution.

I bought all the material. what can I do to prevent my workers from steeling the material. I am not saying that they did or will do, but just in case. They hold a key, and this is the first time they work for me.

Any thought? Thanks in advance.