Compressive Strength is a measurement of a materials resistance to Compressive Stress, which is created by any force applied to the stone mass. In the removable side of dental technology this compressive stress takes place when the completed wax-up of a given prosthesis is flasked, trial packed and then final packed (Hydraulic or Mechanical Axial Press) and pressed (Hanau Flask Press) for curing. Compressive Strength is measured with a device called a TINIUS MACHINE, which creates single directional pressure measured in pounds per square inch, on a sample of the material to be tested. When the material fractures under this increasing load, a measurement is noted and assigned to that tested material.
A certain amount of Compressive Strength is necessary to withstand the pressure from these procedures. However an important balance also needs to be present as there is a relationship between the compressive strength of a material and that materials ability to be broken and removed from the completed flask, model and prosthesis. For example, Crown and Bridge Die Stones of the TYPE IV and TYPE V categories are substantially higher in Compressive Strength (this is not an indication of higher surface hardness nor of general tensile strength) and are generally not used in removable prosthetic flasking processes because they are difficult to fracture and remove.
When flasking in preparation for trial packing, technicians generally follow a rule of using a flasking stone with a lower Compressive Strength than the model on which the denture or partial was originally constructed. Some even ‘create’ their own materials by combining stones and plasters in various ratios to reduce the compressive strength of the flasking materials (the technician should be aware that the combining of materials also changes all of the other physical properties stipulated by the manufacturer). It is always preferable to use a Stone that is designed and manufactured for your specific purpose if you are looking for consistency in performance.
Compressive Strengths of materials can be reviewed and compared by looking at the manufacturers PHYSICAL PROPERTIES CHART, and then make your selections according to the process needs.