Abstract
Excess water in set dental stone decreases its strength. So different methods were used to expel excess water but the compressive strength may be affected by drying technique. The purpose of this study was to evaluate the compressive strength of two types of set dental stone after air, conventional oven and microwave drying techniques. A total of 60 stone specimens (30 specimens made from Silky Rock stone and 30 Zeta stone) were prepared by the aid of an acrylic split mold according to ADA Specification No. 25. Specimens were divided into six groups of 10 identical specimens for each. Then groups were either dried by air, conventional oven or microwave oven. Using Unconfined Compression Machine, the specimens were loaded by a cross head speed of 1 mm/minute till the specimen being fractured. The load required to fracture the stone specimens was recorded and analyzed using analysis of variance followed by Duncan’s Multiple Range Test for the statistical comparisons between drying techniques at a significance level of p< 0.05, and Student’s t–test was used to compare between the two stone types. The results revealed that high significant differences were present between the different drying techniques (p<0.0001) with air dried specimens were significantly stronger than others and microwave dried specimens were significantly stronger than conventional oven dried specimens. Silky Rock (type IV) stone was significantly stronger than Zeta (type III) stone (p<0.001). From this study, it could be concluded that the highest compressive strength can be obtained by air drying of the stone for 24 hours, while microwave drying technique give better results than conventional oven drying technique with the advantage of time saving over the two other drying techniques.
Highlights
They are not directly used as dental restorative materials, gypsum products are important adjunctive materials used in many laboratory procedures.(1) The working model is a replica of oral structures on which an appliance or restoration is made
Rength and abrasion resistance as it will be subjected to the stresses of carving and finishing procedures.(2) A type III dental stone and high strength dental stones are able to withstand most of the manipulative procedures involved in production of appliances and restorations.(3) Gypsum products when mixed with water set to form hard mass, the actual amount of water required for mixing is greater than the amount necessary for the chemical reaction
The water that remains after the completion of the chemical reaction is called excess water which definitely affect the strength of set product as when specimen has been dried the dry strength may be two or more time the wet strength.(3, 4)
Summary
They are not directly used as dental restorative materials, gypsum products are important adjunctive materials used in many laboratory procedures.(1) The working model (cast) is a replica of oral structures on which an appliance or restoration is made. Sometimes it may be inconvenient to wait such a long time, dentist often find it necessary to manipulate the cast as soon as possible after they poured These wet casts possess insufficient strength and surface hardness to withstand the manipulative procedures without being fractured or distorted.(2)
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