Structure and Properties of Gold Foil and Mat Gold

¹ Department of Operative Dentistry, University of Washington, School of Dentistry, Seattle, Washington

Different forms of gold foil, mat gold, and solid gold were studied under the microscope. Selected specimens were subjected to heat and pressure to produce structural changes for comparison with those seen in compacted specimens of gold foil and mat gold. Explanations for the properties of the structures seen under the microscope were taken from the literature and presented in conjunction with photomicrographs that recorded the changes. Evidence of the cubic structure of gold was seen in the etch pits and dendritic crystals of melted gold and mat gold, but none was found in the gold foil. Cohesion was explained in terms of metallic bonding, and the lack of uniform compaction was seen to be responsible for porosity, density, and non-uniform hardness in the compacted specimens. Hardness was discussed in relation to plastic flow, slip, deformation, and strain. Evidence of plastic flow was seen in the polished layer at the surface of prepared specimens. Fracture was induced artificially in a gold-foil specimen, but it occurred naturally between the small mat-gold crystallites. In the compacted specimens, plastic flow was limited to short distances under the condenser nib. The final density was determined by the cohesive and strain-hardening properties of the gold materials, the size and shape of the condenser nib and cavity, and the direction and magnitude of force.