Mechanical Properties of Hydrocolloid and Rubber Impression Materials
Hampson' has studied the elasticity of alginates under compression and has observed that materials with smaller amounts of permanent deformation were more suitable for use in the presence of undercut areas. Schoonover and Dickson2 report the breaking stress, percent strain, and percent set of alginates. Cresson3 suggested the revision of the testing procedure for compressive strengths of dental elastic impression materials that would apply the load at the rate of 18,000 Gm. per minute. Skinner et al.4 and Phillips5 point out the importance of the rate of deformation on the technic of handling alginate hydrocolloids. Warner" also shows that the breaking strength and percent deformation of alginates are functions of the loading rate where the higher the rate, the greater is the strength qualities. Anderson7 shows the recovery from deformation of alginate to be a direct function of time from the start of mixing. Donnison and Docking8 have measured the permanent deformation, stiffness, and toughness of alginates, the latter being determined as a combination of compressive strength and the strain at rupture. The ADA Specification 189 requires the compressive strength to be determined at a rate of 10,000 -+ 2 000 Gm. per minute. Based on an average strength of 7,000 Gm./cm.2 and a specimen '2 inch in diameter, the time required for rupture would be approximately half a minute. Recently, Braden'0 has determined the energy of tearing of one each of mercaptan, silicone, and alginate impression materials as a function of the rate of tearing and the temperature of testing. As the rate of tearing increased and the temperature decreased,
Proposed specification for impression material; synthetic rubber base, dental
This proposed specification is based to a large extent on data reported in a paper by the same authors.1 Test procedures for permanent deformation and strain are modifications o f procedures described in Federal Specification U-I-496a for Im­ pression Material, Hydrocolloidal, Agar Type, Dental; Federal Specification U -I498 for Impression Material, Hydrocol­ loidal, Alginate Type, Dental; and Amer­ ican Dental Association Specification No. 11 for Hydrocolloidal Impression Mate­ rial, Agar Type. Two types of material, silicone base and polysulfide base, are covered in the specification. Although all requirements with the exception of composition are identical for both types, mixing and handling characteristics do differ and the type to be obtained is left to the choice of the purchaser. Determinations of permanent deforma­ tion and strain at 8 to 10 minutes after the material is mixed serve indirectly as requirements that the material set within this period. A requirement for dimen­ sional stability is included since these materials have been recommended for procedures where a considerable delay may occur between making the impres­ sion and pouring the cast. 1. SCOPE AND CLASSIFICATION
Inference and Evidence in Archaeology: A Discussion of the Conceptual Problems
Publisher Summary This chapter dicsusses the general features of scientific inference and explains the way inference has been discussed by archaeologists. It presents an explication of current problems in the various areas: (1) the justification of archaeological conclusions, (2) the concepts of archaeological evidence, and (3) the undeveloped relations between archaeological theory and archaeological evidence. The chapter defines these problems and explains the way to resolve them. To resolve many of the problems surrounding uncertain links between inference and evidence, archaeologists should develop a rigorous model that specifies the way information about the past is transmitted to the present via material remains. The chapter presents the structure of such a model. However, establishing relevant data to support conclusions about past phenomena is only part of the overall process of generating archaeological knowledge. Some of the processes responsible for producing the material remains that archaeologists manipulate in assembling evidence to support claims about past phenomena are described in the chapter.