A new investment material for the pressing loss wax technique for dental glass ceramics. It has been found that the addition of fillers to a magnesium phosphate investment, specifically metal oxides with elevated melting points ranging from 1800 to 2800° C., provides a protection barrier against the reaction between the high alkaline content of the glass ceramic and the investment during the pressing process in the range of 800 to 950° C. Specifically, it has been found that the addition of aluminum oxide of about 2 to 5 percent of the total dry mix in combination with any of the zirconium oxide, yttrium stabilized zirconium, titanium dioxide and boron nitride in proportions of about 3.5%, enhances the barrier against a surface reaction and improves the thermal properties of the investment.