The Collaborative International Dictionary of English v.0.48:

Assayer \As*say"er\, n.
   One who assays. Specifically: One who examines metallic ores
   or compounds, for the purpose of determining the amount of
   any particular metal in the same, especially of gold or
   silver.
   [1913 Webster]

WordNet (r) 3.0 (2006):

assayer
    n 1: an analyst who assays (performs chemical tests on) metals