Take a scraping of the gold piece.
Weigh it using an accurate metric scale. Use a sample of 250 mg.
Wrap in lead foil. Add a small amount of silver scrapings. The silver will bind to the gold, while other metals and impurities are extracted from it.
Place in a cupel, which is porous pot used specifically for assaying metals, and is able to withstand the necessarily high temperatures of the assaying procedure, also known as cupellation. Base metals will be removed during this procedure.
Melt in laboratory oven at a temperature of 1100 degrees C.
Allow gold/silver button to cool.
Dissolve silver from gold with 35 percent strength nitric acid.
Weigh resulting gold.
Subtract the new weight from the original weight and compute the percentage. At 100 percent, you have 24 karat, or pure gold. Anything less is considered against 24K.