title: Alterations of surface hardness with gypsum die hardeners.
abstract: BACKGROUND
  Die stones require abrasion resistance, dimensional stability with time, and high surface wettability material properties.
  
  
  OBJECTIVE
  The purpose of this study was to compare the surface microhardness (Knoop) of 4 contemporary gypsum materials with and without surface die hardener.
  
  
  METHODS
  Materials used were a Type III stone (Microstone) and 3 die stones (Die-Keen, Silky-Rock, and ResinRock). Die hardener was cyanoacrylate (Permabond 910) or Clear Coat. Specimens of stone were hand mixed with distilled room temperature water and vacuum spatulated according to manufacturer's directions. Five cylinders (15 x 15 mm) per group were poured, using vibration, into phenolic ring molds positioned on top of a glass slide. The face of each specimen was polished with 2400-grit Al2O sandpaper. One face of each of 5 specimens/material was coated with cyanoacrylate; 5 specimens/material were coated with Clear Coat, air thinned and dried; and 5 specimens/material had no treatment (control). Knoop hardness (kg/mm2) readings were made on each face (5 readings/time point) 3, 12, and 24 hours after pouring. An ANOVA procedure with post hoc Tukey tests were performed (alpha=.05).
  
  
  RESULTS
  Microhardness did not vary between 3 and 24 hours for any material (P>.05). Microstone had significantly lower surface hardness (P<.0001) than the die stones. Specimens coated with die hardener had lower hardness values (P<.001 in all cases)
  
  
  CONCLUSIONS
  The 3 types of die stones evaluated in this study did not differ significantly in surface microhardness. Under these conditions, die hardener coatings reduced the surface hardness of the gypsum material.
authors.affiliation: University of Iowa, College of Dentistry, Iowa City, USA.
authors.forename: Paul E
authors.initials: PE
authors.lastname: Harris
authors.forename: Scott
authors.initials: S
authors.lastname: Hoyer
authors.forename: Terry J
authors.initials: TJ
authors.lastname: Lindquist
authors.forename: Clark M
authors.initials: CM
authors.lastname: Stanford
mesh_terms: D000704:Analysis of Variance; D002133:Calcium Sulfate / Q000737:chemistry*; D020099:Coated Materials, Biocompatible; D003487:Cyanoacrylates; D006244:Hardness; D007659:Ketones; D003765:Models, Dental; D012117:Resins, Synthetic; D013499:Surface Properties
chemical_list: D020099:Coated Materials, Biocompatible; D003487:Cyanoacrylates; D007659:Ketones; D012117:Resins, Synthetic; D002133:Calcium Sulfate