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<p>[QUOTE=&quot;Conder101, post: 4644143, member: 66&quot;]The &quot;blackboard&quot; is called a touchstone.&nbsp; It is a fine grain stone (sorry I don&#039;t know the exact mineral) and it acts like a very fine sand paper to scrape&nbsp; a little bit of silver off the edge of the coin leaving a streak of metal on the stone.&nbsp; You then put s drop of a known concentration of acid on the strek&nbsp; to see if it reacts and if so how fast.&nbsp; There are different acid concentrations to determine the fineness of the metal.&nbsp; Touchstones withthe proper acids can be used to determine the fineness of either gold or silver.&nbsp; In some cases they will also make streaks along side the test streak using testing needles which are small rods of metal of known fineness.you put streaks of several different fineness alongside the streak in question and put the acid on all of them and observe which needle streak reacts the same and the test streak.[/QUOTE]</p><p><br /></p>

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