Measuring diameters

[Andrew McCabe]

I just got a new digital calipers (model as per pic below) and wanted to go through my collection and update my coin descriptions (also online) with diameter measurements.

Is there a standard way to measure? 9pm-3pm? Largest dimension? Largest and smallest? 9pm-3pm and also 12-6pm? Average of these two? Outside coin diameter or die circle? Both? Others must have experience in this dilemma.

I thought to consistently measure the largest single outside dimension. This probably requires the least explanation, though it's going to make oblong coins sound large, and round coins sound small. Or else 9-3 and 12-6. Or average of these two? I'm lost. Notation in simple terms is key - do anything complex and one doesn't want to spend two lines describing the diameter of a perfectly round coin.

ADMIN NOTE:  FORVM SELLS CALIPERS - https://www.forumancientcoins.com/catalog/roman-and-greek-coins.asp?vpar=861&pos=0#Coin-Collecting-Supplies

[maridvnvm]

I always measure the largest single outside dimension as a single measurement without further explanation but where the coin is obviously oblong / ovoid I give the smallest outside dimension to with an explanation.
Regards,
Martin

[Andrew McCabe]

I always measure the largest single outside dimension as a single measurement without further explanation but where the coin is obviously oblong / ovoid I give the smallest outside dimension to with an explanation.
Regards,
Martin

Thanks Martin - I guess its indeed probably easiest just to quote the largest only, and by exception largest and smallest for odd-shaped coins.
Andrew

(NB I notice that the same calipers is available on Forvm for $18.)
https://www.forumancientcoins.com/catalog/roman-and-greek-coins.asp?param=00523q00.jpg&vpar=861&zpg=8478

[quadrans]

Hi all we measuring the smalest ang largest two and not average like: diameter: 13-15mm because if the coin are very assimetric then the bigest size are fals info
regards quadrans

[areich]

That's what Martin said and what most people do, if the coin is oblong, give largest and smallest measurement, if it's more or less round just the largest.

[PtolemAE]

I just got a new digital calipers (model as per pic below) and wanted to go through my collection and update my coin descriptions (also online) with diameter measurements.

Is there a standard way to measure? 9pm-3pm? Largest dimension? Largest and smallest? 9pm-3pm and also 12-6pm? Average of these two? Outside coin diameter or die circle? Both? Others must have experience in this dilemma.

I thought to consistently measure the largest single outside dimension. This probably requires the least explanation, though it's going to make oblong coins sound large, and round coins sound small. Or else 9-3 and 12-6. Or average of these two? I'm lost. Notation in simple terms is key - do anything complex and one doesn't want to spend two lines describing the diameter of a perfectly round coin.

these make it very easy to note both largest and smallest. no harm in having more data  :-)
Ptolemae

[Robert_Brenchley]

I give one as long as the coin's more or less round. If the coin is noticeably oval, I give both largest and smallest.

[Lloyd Taylor]

and don't forget this sort with two distinct dimensions... measuring the diagonal (the largest dimension) would not be very helpful... "horses for courses."

[PtolemAE]

and don't forget this sort with two distinct dimensions... measuring the diagonal (the largest dimension) would not be very helpful... "horses for courses."

I see a lot of Ptolemaic bronzes with casting sprues so the coin may be fairly round itself but it can have one or two squarish extensions sticking out one or two sides - and they are definitely parts of the coin itself.  The difference between the measured size including the sprues and the 'round coin' is often substantial so two measurements tell the whole story.  Can't see any other fair way to describe a coin that's has such peculiar shape.  I simply record both measurements. 

Ptolemaic coins are often beveled as well but I measure at the 'wider' edge, usually the edge of the reverse as the obverse can often be significantly smaller.  Some Ptolemaic bronzes are beautifully made and almost round, with sprues removed during manufacture, but some are a lot more carelessly produced. 

Beveling and sprues are just facts of life with many of them.  The flans were cast, of course, and often in moulds that allow for easy removal by being beveled.  The mould pits for the discs are probably always connected by sprue channels.  Not sure why the sprues are so prevalent on some types and on others the edges are all cleaned up and smooth because the sprues were present when they came out of the moulds as 'casting trees'.  It's almost a diagnostic identifying feature for some types that one or two sprues (depending on how the casting tree was laid out) will be present - if not, one suspects the coin is fake.  Yes, there are some Ptolemiac bronze fakes - fortunately not too many and mostly recognizable as in electrotypes, etc.

PtolemAE

[Andrew McCabe]

My rule of thumb is two millimetres long.

If a coin, regardless of size, is within 2mm of being round (e.g. 21mm wide by 22.5mm high) then I list its largest dimension as a single number, in whole millimetres at or just below the largest dimension. For example I would probably record a 21mm x 22.5mm coin as being 22mm.

If there is 2mm or more difference, then I describe more. This might be length x breadth, or it might be a diameter combined with a comment on casting sprues, and/or I might include a comment on the general shape of the coin. Free-format.

I avoid fractional millimetre reporting, except for coins under 20mm that are really round, where I sometimes report to a half millimetre.

One has to consider the purpose of recording diameters. It allows one to suggest die-diameters by visual estimate and thus to allow comparison and die-studies between other coins in a series. It allows you to make positive statements such as "broad thin flan" for sereis which are generally controlled by weight. It can be an instant alternate to guaging a denomination. For all these sorts of purposes, the 2mm rule is generally sufficiently precise, as that is about the uncertainty in flan-casting moulds and in die-size diameter for any one coin series. So it is generally pointless to be any more precise, because if the romans and greeks could not make coins more precisely, then there seems little point in recording them more precisely.

[SC]

Andrew,

As you say it depends on what you are measuring. 

All the methods discussed above measure the flan - which as you note were generally not controlled very closely since flan fabrication and/or striking could cause variation.

Die diameter on the other hand is usually measured using a different technique.  Instead of the outside diameter the PRD is measured (Pearl Ring Diameter).

The PRD is simply the diameter across the ring of pearl dots that forms the outer border of the die.  To be exact it is usually measured from high point (or apex) to high point of the dots.

Of course this technique can be difficult to do if the coin is too corroded, or if there is less than 180 degrees of the pearl ring present on the coin, or if the coin type was struck without a pearl ring.

Shawn