After drying, the trays of discs were supported in custom made silicone holders (Fig. ), covered in 2 mm silicone sheet and vacuum sealed in Nylon bags (Fig. ) before iso-pressing in an Autoclave Engineers 75 mm diameter wet bag laboratory isopress at 15,000 to 30,000 p.s.i. (105 -210 MPa) to increase the density and strength of the discs. The silicone holders contained metal inserts to improve the flatness of the bottom faces of the discs. The trays were then inverted onto refractory batts (Fig. ) so that the flat faces of the discs were uppermost thus avoiding reaction between this sample surface and the refractory.