Physical Characteristics of Coins

It is important to recognize that a scanned image posted on the web does not give any information about the physical characteristics of the coin. Indications such as "200% of real size" or whatever I often see is not a useful measure since all of us don't use the same size of computer screen. If the dpi (dots per inch) of scanning is given then from the size of the image the diameter can be worked out. However it is best to give a diameter/thickness in mm and if available the weight in grams to to best available precision.

• Size Some scan the coin kept next to say a US coin or a straight edge of a ruler. Both however distractions from the coin image on a web site. It is better to give the size measured to an accuracy of 0.1 mm with a pair of calipers or even estimated using a good ruler. I was able to buy a 0-100 mm digital Calipers on Ebay for $40 which is pocket size and fast to use. It measures to an accuracy of 0.02 mm, more than necessary for most coins. The US MInt claimed 0.2% accuracy in diameter which probably represents modern minting. If the coin is elongated measure both major and minor axis of the coin. Note also the thickness of the coin is not usually visible in a scanned image of the coin face. It is also particular large and variable for most SouthAsian Dumps.
  For coins which are round I specify the diameter and for other odd shapes need a maximum and minimum diameter or width. For elliptical coins I give the maximum and the axis ratio with the elliptical shape description.
  I measure the thickness a few times and give a rough estimate of the average of the measurements.

  I have found this digital caliper sensitive to humidity. Although it works perfectly in a dry climate, it need to me kept in an Air conditioned room in a tropical climate like in Lanka. I does recover from flashing random numbers in a humid environment.

• Weight The weight of the coins often says a lot about it's authenticity. Most genuine coins are well within a few percent of their nominal weight unless differences are noted in reliable catalogs. The US Mint claimed 1.5% accuracy in weight which probably represents modern minting. A coin which is more than 10% under-weight is at best a contemporary forgery. It is best to be able to measure to 0.01 gram accuracy. However a good scale is unfortunately very expensive, prices IMHO probably maintained by the drug trade. The cheap Indian brass scales sold on ebay (2000 May for $14) are useless. I found the weight wrong by up to 20%. A dealer once sent me a weight of a silver crown sized coin from a diet Scale which was 2/3 of the actual weight. Another just said 1/2 ounce clearly also from a kitchen scale, maybe not understanding my request. One ounce is 28.3495 grams and the coins under question is expected to be about 1/3 ounce. A post-office scale reads to 0.1&ounce or nearest 1.4 grams is also not of much use for coins. I use it for items over 100 grams which is outside the range of most high precision scales. A lever balance I purchased (1998 Oct for $10) which claimed 0.02 gram precision was time consuming and almost impossible to use to that accuracy. I purchased a digital scale on ebay (2000 July for $57) which is pocket size (17mmx155mmx75mm) and fast to use. Unfortunately gives only 0.1 gram accuracy to 50g. Recomended by Steve Album, the least expensive scale ; the Dillon D-Terminator which (75gx0.01g) was $179 including postage, but also a heavy (3 lbs) desktop model used for measuring gunpowder for making one's own bullets. In 2001 Sept I purchased a Myweigh MX-50 (50gx0.01g 21mmx135mmx83mm 160 grams net) for $106. which is about the same physical size of the Tanita. Tested against Standard weights in the Chemistry Lab at CMU I find that it satisfies the claimed accuracy of 0.01grams over the full range. It comes in a hard plastic case which is opened to operate the scale. Works on 4 AAA Batteries included and the 50gram calibration weight also provided was also within the expected accuracy. But the MX-50 manual says " Although this scale is designed to be quite durable, try to avoid rough treatment as this may permanently damage the internal sensor. Dust, dirt, moisture, vibration, air currents and proximity to other electronic equipment can all cause an adverse effect on the reliability and accuracy of your scale. Do not place anything that weighs more then 50 grams on the scale or you may permanently damage the scale and void your warranty. Its durability needs to time tested. I have had no problems with it so far. I guess it would get tested on the street where it will probaly see more abuse than in the hands of coin collectors :-). Refer to an interesting dealer review
  For Ancient and most Medieval coins there is no known "Legal Weight" or "prescribed Weight" For coins that are common there are "Average weight" which have sometimes been published. For most early colonial coins made by hand there is a legal weight specified by the contract and it is useful to compare it with the actual weight of coin listed. For modern struck coins the variation in weight in the manufacture is too small to be important and only the legal weight is given.

  The 1914 Schulman Auction catalog of the Grogan collection gives weights for nearly all of the coins which is very useful. It is unfortunate that most modern Auction catalogs very rarely give this information when digital scales and computer databases should make that task much simpler today.

• Die Axis The alignment of the face on the coin on the two sides when orientation of both sides is well defined is sometimes useful to spot a careless forgery. Most coins from regular mints have either of two standard alignments. In the 0° medal alignment the top of both sides of the coin is on the same point on the rim. Put your finger on the top of the obverse design. Turn the coin over. Measure angle clockwise from the top of the reverse design to the location of your finger. In the 180° coin alignment the top of one side is the bottom of the other face of the coin. I will someday make this entry for all the coins on this website. It is easy and sufficient to estimate by eye to 30° (hour on the clock-face), but maybe possible to 15° with some care. Using negative for and anti-clockwise rotation I plan to put entries from -180° to +180° since it is the absolute value that is useful to compare.

• Density If the volume of the coin can be estimated with sufficient accuracy it's density computed. A Gold pagoda has a weight of 3.4 grams corresponding to a volume of under 0.2 cc. To measure the volume to a 2% accuracy using the change in weight when immersed in water, that change needs to be measured to 0.004 cc or 4 mg precision. I suspect it is simpler to measure the displacement of water along a capillary tube to a better accuracy.
  Basic idea. A bottle nearly full which has stopper with fine glass tube attached. When the stopper is inserted liquid reaches up to some distance along tube since the bottle is more full.. Put the coin into bottle and insert stopper liquid goes much higher up tube The difference gives volume of coin with some calibration/correction for capillary action. Will need a shallow but wide mouth bottle to put a coin in and a well fitting stopper which can be inserted to precisely the same place without spilling any liquid when replacing stopper.
  I have not yet included density in the list of specifications.

• Alloy
  

  Name of Alloy	Composition
  Sterling Silver	Ag 92.5% ; Cu 7.5%
  500 Fine Silver	Ag 50% ; Cu 50%
  Cupro-Nickel	Cu 75% ; Ni 25%
  Nickel-Brass	Cu 79% ; Zn 20% ; Ni 1%
  Aluminum	Mg 3-4% ; other alloys < 3.2% ; Al > 93%

• Assay Analyze and determine the purity of metal such as Gold or Silver. See discussion.