A 90% silver quarter contains 0.1808479 of an ounce of silver, so 5.5 of them adds up to one troy ounce. Since you can't have half of a coin, rounding up means it takes six silver quarters to make more than a troy ounce. To add up to an ounce of silver, it takes three pre-1965 half dollars, six quarters, or 14 dimes.