A simple Google search will tell you all that you will need to know.
The following is from Contenti's website (shown below) including
Delft Clay Casting Set
The Delft Clay Method is a highly refined form of sand casting that
substitutes genuine Delft Clay from Holland for ordinary casting
sand. The process was invented by Hans Karreman, a Dutch goldsmith.
Delft Clay is much finer than casting sand, providing remarkably
sharp and highly detailed castings in gold, silver, copper-based
alloys, pewter, etc. Castings require very little finishing work.
Once you have cast your piece, the Delft Clay can be re-used. Since
this is a cold-molding process, molds can be made from patterns
composed of wood, wax, epoxy, plastics, and, of course, metal.
Because this is a type of sand casting, it is best suited to
producing castings without pronounced undercuts. A Delft Clay mold
is created in these steps:
1. Fill and compact the Clay into one half of the aluminum ring
2. Remove excess clay with a straight edge (e.g. a flexible steel
3. Press your original pattern halfway into the clay.
4. Brush Mica Powder or talcum powder over this surface of the mold
5. Install the other half of the aluminum ring frame, fill with clay
6. Open the ring frame and remove the pattern
7. Carve out a pouring channel and vents
8. Pour in molten metal
The Delft Clay Casting Kit includes 4.4 lbs. of Delft Clay, a 60mm
dia. Aluminum Ring Frame, and detailed instructions. A 100mm
diameter Aluminum Ring Frame is available separately. View the Delft
Clay video for complete step-by-step instructions on the Delft Clay
Method. Made in Holland.