There are a lot of tutorials and articles (searched google) about baking air-drying clay to speed it up, but I don't see any mentioning Das specifically. I know Das makes an air drying and an oven baking clay as 2 different products though.
Their page for the air-drying clay says drying should take 24 hours for every centimeter of clay thickness. Have you allowed enough time for full drying?
Putting it in the oven at a low temperature may speed up the drying time, but it could also cause cracking or other issues since the clay's manufacturer does not appear to list baking as a typical step with this clay.
As far as getting it to be harder, once it's fully dry the firmness you get should be the firmness it's designed to have. It is not a firing clay, so it will not likely get as hard as those.
Comments
Their page for the air-drying clay says drying should take 24 hours for every centimeter of clay thickness. Have you allowed enough time for full drying?
Putting it in the oven at a low temperature may speed up the drying time, but it could also cause cracking or other issues since the clay's manufacturer does not appear to list baking as a typical step with this clay.
As far as getting it to be harder, once it's fully dry the firmness you get should be the firmness it's designed to have. It is not a firing clay, so it will not likely get as hard as those.
/Chris