Potter produced approximately 200 - 300 lead testers from 1940 to 1975.  The testing method is the same as many modern testers.  In this case a small, 1/8" steel ball at the end of a piston (located underneath the cast iron body of the tool) is pressed lightly against a lead sample by the screw on the top.  A standard one pound weight is applied.  the weight forces the ball into the lead sample and the operating lever begins to go down.  The distance to which the lever goes down is measured against a brass scale.  That measurement translates to given hardness.  Potter provided an approximate correlation between the numbered result and common alloys.    The directions for use may be found below.

Some technical information:  The brass scale has mill marks graduating from "0" to "10" but displaying only the even numbers.  The distance between "0" and the first mill mark is 1/2 inch.  Subsequent marks follow every .115 inches.


To operate the Tester, place an ingot or a bullet on the anvil and secure it firmly with the clamping screw, at the same time being careful to hold the operating lever at the top of its travel so that the measuring point will not come in contact with the lead.  Now lower the lever gently, and if the bevel on the lever does not line up with the zero on the scale, loosen the scale binding knob and slide the scale up or down until it registers zero.  The weight, which is furnished with the tester, should now be suspended from the notch in the end of the lever, and the position taken by the lever read on the scale, which will indicate the hardness of the material.  In selecting an ingot or other piece of lead to be tested, care should be exercised to select a SMOOTH spot to come in contact with the ball.  If the surface is filed smooth, greater accuracy will result.  However, it must be remembered, that lead alloys are seldom entirely uniform, and variations will be noted at various points on the same piece tested.  Therefore, and average of 3 or 4 readings should be taken to obtain the hardness of the piece tested.  Due to the relative softness of lead alloys, the steel ball will continue to slowly sink into the metal if the weight is allowed to remain on the lever.  For greatest accuracy, the reading should be made after a uniform length of time has elapsed after applying the weight.  The reading may be taken the instant the lever comes to rest, or after a period of from 10 to 20 seconds.  Whichever method is selected it should be used each time for accurate comparisons.
     A harness of 5 on the scale indicates that the lead is approximately the same hardness as 18 parts of lead and 1 part of tin, while 6 indicates a hardness of about 25 to 1.  The most practical way to use the Tester is to mix the alloy in the usual way and note its hardness on the scale of the Tester.  The next batch of metal can then be tested and compared with the hardness of the previous batch, and if found to be too soft or too hard, additional material can be added to bring it to the desired hardness.
     The following table is a guide for using the Tester:
2-1/2 Scale Reading is approximately 10 to 1
4         Scale Reading is approximately 15 to 1
5         Scale Reading is approximately 18 to 1
6         Scale Reading is approximately 25 to 1

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