George Dickson

Hardening Shrinkage and Hygroscopic Expansion of Composite Resins

Polymerization shrinkages of various restorative resins were measured. The specimens stored in water expanded, but most did not recover sufficiently to countervail the losses from polymerization.

Physical and Chemical Properties and Wear

Although the title of this paper is very broad, the discussion will be limited to two areas, dimensional changes and wear. Neither dimensional changes nor wear can be isolated from other physical and chemical properties. Both depend, of course, upon the components of the composites as discussed in later papers. Also it is probable that the stresses produced by dimensional changes may play a significant role in the wear of composites.

Elastic Constants of Dental Amalgam

men was coupled with resin 276-V9.* Signals representing the first transit and subsequent echoes from a second quartz transducer on the opposite end of the specimen were amplified with a wideband amplifier, and transit times were determined by an oscilloscope with a dual trace unit which also displayed the input pulse. Pulse repetition rate and oscilloscope sweep initiation were controlled by a wave-form generator. Transit times ranged from 2 to 11 gssec., depending on the length of the specimen and on whether longitudinal or transverse waves were used. Specimens were made by condensing dental amalgam,t certified to comply with American Dental Association Specification No. 1 for dental amal-

Comparison of Brinell and Vickers Hardness Tests on Dental Casting Gold Alloys

The relationship between Vickers and Brinell hardness numbers of dental casting gold alloys was found to be linear; the addition of 19 to the Brinell number approximated the Vickers number fairly well. The coefficient of variation for the Brinell test was about 1 % less than that of the Vickers test. Effects of reading error, sample inhomogeneity, and other factors on the precision of the Vickers values were evaluated.

Chemical Composition of Dental Gold Casting Alloy and Dental Wrought Gold Alloys

The chemical composition of dental casting gold alloys and dental wrought gold wire alloys was determined by use of a modification of a previously described X-ray emission method. Since 1928 and 1932 there has been little change in either type of alloy; there has been a decrease in platinum content in both, and 1 % nickel is now found in wire alloys.

Application of Diffraction Gratings to Measurement of Strain of Dental Materials

Strains occurring on application of loads to small specimens of dental materials were determined by measuring the changes in the angle of the beams of light diffracted from gratings ruled on the specimens. The method gives a rapid response to stress and is effective over very short gauge lengths.

Dimensional Changes of Dental Amalgam Associated with Strain Release in the Silver Alloy Particles

It is generally recognized that the dimensional changes of amalgam on setting can be altered significantly by heat treatment of the amalgam alloy particles at 1000 C. Waterstrat has shown (J. Research NBS, 64A: 317-23, 1964) that heat treatment results in relief of strain in a silver-tin alloy. Thus it appears that dimensional changes are related to the amount of strain in the alloy particles, and that dimensional changes of amalgam may be at least partially due to strain release that occurs when mercury diffuses into the alloy particles. An experiment was designed to determine the magnitude of dimensional change associated with strain release. Particles of a commercial alloy complying with the American Dental Association specification for alloy for dental amalgam were placed in a steelmold (without the addition of mercury) and subjected to pressure. The applied load was equivalent to approximately 965 X 106 N/M2 (140,000 psi) on the plunger of the mold; pressure on the particles