The Collaborative International Dictionary of English v.0.48:

Assay \As*say"\, n. [OF. asai, essai, trial, F. essa. See
   Essay, n.]
   1. Trial; attempt; essay. [Obs.] --Chaucer.
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            I am withal persuaded that it may prove much more
            easy in the assay than it now seems at distance.
                                                  --Milton.
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   2. Examination and determination; test; as, an assay of bread
      or wine. [Obs.]
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            This can not be, by no assay of reason. --Shak.
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   3. Trial by danger or by affliction; adventure; risk;
      hardship; state of being tried. [Obs.]
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            Through many hard assays which did betide.
                                                  --Spenser.
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   4. Tested purity or value. [Obs.]
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            With gold and pearl of rich assay.    --Spenser.
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   5. (Metallurgy) The act or process of ascertaining the
      proportion of a particular metal in an ore or alloy;
      especially, the determination of the proportion of gold or
      silver in bullion or coin.
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   6. The alloy or metal to be assayed. --Ure.
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   Usage: Assay and essay are radically the same word; but
          modern usage has appropriated assay chiefly to
          experiments in metallurgy, and essay to intellectual
          and bodily efforts. See Essay.
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   Note: Assay is used adjectively or as the first part of a
         compound; as, assay balance, assay furnace.
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   Assay master, an officer who assays or tests gold or silver
      coin or bullion.

   Assay ton, a weight of 29,1662/3 grams.
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The Collaborative International Dictionary of English v.0.48:

Assay ton \Assay ton\
   A weight of 29.166 + grams used in assaying, for convenience.
   Since it bears the same relation to the milligram that a ton
   of 2000 avoirdupois pounds does to the troy ounce, the weight
   in milligrams of precious metal obtained from an assay ton of
   ore gives directly the number of ounces to the ton.
   [Webster 1913 Suppl.]