A method of determining an amount of impurities that a contaminating material contributes to high purity silicon comprises the step of partially encasing a sample of high purity silicon in the contaminating material. The sample encased in the contaminating material is heated within a furnace. A change in impurity content of the high purity silicon is determined after the step of heating, compared to an impurity content of the high purity silicon prior to the step of heating. A furnace for heat treating high purity silicon comprises a housing that defines a heating chamber. The housing is at least partially formed from low contaminant material that contributes less than 400 parts per trillion of impurities to the high purity silicon during heating at annealing temperatures for a sufficient period time to anneal the high purity silicon, and the furnace contributes an average of less than 400 parts per trillion of impurities to the high purity silicon under the same heating conditions.