The effect of investment material and ceramming regime on the surface roughness of two castable glass–ceramic materials

Objective: To assess the surface roughness (Ra) of two castable glass–ceramic materials, cast using four different investment materials and employing different ceramming regimes. Methods: Forty discs, each 12 mm diameter and 2 mm thick were produced from two castable glass–ceramic materials, one a glass ionomer derivative based on 1.5SiO2–1Al2O3–0.53P2O5–1CaO–0.67CaF2 (LG112), the other a fluorcanasite material based on 0.60SiO2–0.05K2O–0.10Na2O–0.15CaO–0.10CaF2 (SG3). The discs were made using four dental investments, two gypsum-bonded (Whip-Mix Cristobalite (WMC), Degussa California (DC)) and two phosphate-bonded (Fujivest Super (FS), Techceram Glass (TG)). A Ni–Cr metal/ceramic alloy (Wiron 99) was also used as a control. Each investment was used to produce 10 discs for each glass–ceramic material, five left in the as-cast, glass state, the other five cerammed. Ceramming was carried out either within the investment mold or after de-vesting from the investment mold. The discs had their surface roughness values measured using a stylus, surface contact measuring instrument. Results: For the Ni–Cr alloy the phosphate-bonded investments produced discs significantly smoother than those produced by the two gypsum-bonded investments (p<0.05). The FS investment produced the smoothest discs (p<0.05), but there was no significant difference between the discs produced by the two gypsum-bonded investments (p>0.05). For the SG3 material no significant differences were seen between the as-cast glass and cerammed discs (p>0.05), with the WMC investment producing the smoothest discs (p<0.05), and the TG investment the roughest (p<0.05). The LG112 material showed a significant difference between glass and cerammed discs for the gypsum-bonded investments (p<0.05) but not when using phosphate-bonded investments (p>0.05). With LG112 the WMC investment produced smoother discs than the DC and TG investments (p<0.05). The DC investment produced rougher discs than the two phosphate-bonded investments (p<0.05). Significance: The selection of investment material can have a significant effect on the as-cast surface finish of castable glass–ceramic materials. A gypsum-bonded investment material gave smoother surface finishes compared to phosphate-bonded investments. The ceramming process significantly increases surface roughness.