Abstract
A study on the effect of humidity on the surface hardness of plaster dies
Dr. MR. Sabouni* - Dr. H. Esmaili** - Dr. KH. Shamsian***
 *- Assistant Professor of Prosthodontics Dept. – Faculty of Dentistry – Mashhad University of Medical Sciences.
 **- Assistant Professor of Community Medicine Dept. – Mashhad University of Medical Sciences.
***- Manager of Jahad Dental Clinic of Mashhad.
Background and Aim: Ditching plaster die is a very important part in fabricating fixed prosthetics. Some clinicians humidify the plaster die to simplify the procedure. The objective of this investigation was to assess the effect of humidity on the surface hardness of plaster die.
Materials and Methods: In this experimental study, American-made type 4 gypsum was used according to the manufacturer’s instructions. 70 dies were prepared and were divided into seven groups of 10, 24 hours after pouring the gypsum. The hardness of the control group (first group) was immediately examined. The second, third and forth groups were dipped into water for 10 seconds and the fifth, sixth and seventh groups for 10 minutes. The hardness of the second and fifth groups was assessed immediately, the third and sixth groups after 5 hours and the fourth and seventh groups after 24 hours of being removed from water. Statistical analysis was performed using one-way ANOVA, Duncan and t-tests.
Results: The duration of humidification and dehydration affected the surface hardness of plaster dies. Maximum hardness was observed in the control group (no humidification) (P< 0.001). Surface hardness after humidification for 10 seconds without dehydration revealed the second highest values, after the control group (P< 0.001). Hardness was minimal when the humidifying time was 10 minutes without dehydration (P< 0.001).
Conclusion: Humidification of plaster dies can decrease the surface hardness which is irreversible.
 Key words: Plaster die – Surface hardness - Humidity - Gypsum