Excavator clay, soapstone, and Desert Tortoises

[CandyAss]

I have 3 hatchling Mojave Desert Tortoises that I'm trying to keep entertained and awake through the winter. I have them on a mixture of coco coir, reptibark, and on the basking end Zoo Med Excavator Clay.
They really love the clay, but it seems like it might be drying them out. Perhaps I misinterpreted the intended use? Does it have to be mixed with water and formed, or is it fine dry? They love digging in it, but I'm worried it's pulling moisture from them. Has anyone used this?
I was considering using a 10×12 piece of soapstone under their basking lamp. Their enclosure is on a covered patio, and I have a ceramic heat bulb for overnight use, and that's been keeping them toasty so far. I live in southern California, so it doesn't get extremely cold, but high 20's isn't unheard of in my area every once in a while. Soapstone retains heat, so my hope is that it could help keep their enclosure warm, or act as a backup if there's a power failure. Is there any reason why soapstone wouldn't be advisable?
I have cared for tortoises before, but never at such an early stage. They're only about 4 weeks old, so any advice is welcome and appreciated!

 

[Maro2Bear]

Greetings..

I just took a look at the instructions for the clay as well as some real-life uses by other reptile keepers. Looks like you are to mix with water, get a thick consistency, form tunnels and hides...etc. one person used small balloons to form the cavern, piled on the clay, let it dry, then popped the baloons to have a nice hide.

More info

 

[CandyAss]

I guess I interpreted that part about "letting them do all the work and dig their own burrows" as putting the substrate in straight from the bag. There isn't anything in the instructions to indicate that is harmful, but I'll err on the side of caution and mix in some water while they're having their soak. Santa Ana Winds are forecasted to start tomorrow, and those conditions will dry out a loaf of bread in seconds (lessons learned while camping!), so I don't want to make life any harder on them with improper substrate.