I like the HirstArts stuff (I have most of the Egyptian molds) but they're a lot of work. From my experience with Durham's, you might want to consider an alternative. It's pretty dense when it's dry, particularly in those thicknesses. I'm using dental plaster at the moment and it's great to work with. I just hope I can easily source some more when it's gone. Don't use Woodland Scenics' lightweight plaster, though. Nice material, but it's a bit too 'insubstantial' for this type of modelling - too 'airy'. Works fine for their own rock molds, otherwise.