Forgery Detection

From:  Robert Kokotailo 
Date:  2001 Feb 27, 10:10pm
Subject: [Moneta-L] Re: Forgery detection

There certainly is a lot of confusion about flow marks, but I will try to
explain what it really means.

First, the coin illustrated does not appear to be cast, but rather is
almost certainly die struck.  Unfortunatly, what is being refered to on it
as flow marks are not (although they look very similar).   They are in fact
areas where the metal has warped due to stress, outside of the area of
contact between the die and flan, which is a good sign the coin is die
struck. 

What flow marks really are, are lines left on the metal as it flows into
the die during striking.  They will only be visible at the points where the
metal actually has to "flow" around corners to fill the die, and thus one
sees them at the corners of the various devices on the coin.  The term
stretch marks might describe them better.  They will generally not be
visible on open flat areas of the flan, and cannot be visible where there
is no contact between the die and the flan.  

Flow marks come in various sizes from very visible, to microscopic too
small to show on a standard coin image.  If you look on the obverse at 9
o'clock, between the letters "NA" and the edge of the coin, you will see
what are very large flow marks where the metal has stretched against the
die on striking.  These types of very heavy flow marks are commonly seen
between the lettering and rim of many coins, but one should be very careful
about using them for authentication, as they are so large that a good
casting can pick and duplicate them up.

For authentication work, you are looking for flow marks that look almost
identical to these, but are about 1/50 the size and will be commonly seen
surrounding many features on a coin.   You will need at least a 10 power
magnification to look for them. but please note that they will not always
be present around all features, and there are a number of reasons why they
may not be visible at all on many genuine coins.

Because the flow marks are very very small, for coins with even average
wear (say grading VF or less) you run into a problem in that most of them
will be worn off.  Only in highly protected areas (such as the insided of
letters, where no wear occures will they usually be visible to some degree
even on very worn coins.  

Unfortunatly, this will not be of much use on most bronze coins, because
any significant disturbance of the surfaces, which remove even a slight
amount of metal, will destroy these microscopic flow marks.  Since most
bronze coins have been patinated (a process that chemically alters the
surface of the coin) and/or lightly corroded (a process that removes the
original surface of the coin), none will generally be visible.

Any coin that has been polished, abrasively cleaned, or cleaned with any
kinds of harsh chemicals, may have had enough of the surface metal removed
to also remove all of the smaller flow marks.

IT IS VERY IMPORTANT TO REALIZE, as discussed above, to realize their
absence does not prove a coin is cast or a fake.  Unfortunatly, they will
be present on any die struck coin, including die struck fakes (Slavei's
have excellent flow marks).  They are a tool to be used with many other
tools in determining a fake from a genuine coin, but by themselves cannot
be used to authenticate or condemn a coin.

There is no magic bullet, no single diagnostic feature that will tell you a
coin is genuine or fake.  There is just long experience and study, and a
desire to learn.



From:  Robert Kokotailo 
Date:  2001 Feb 28,  3:16pm
Subject:  Re: [Moneta-L] What is die axis???

Die Axis is an interesting subject because as records for it improve
it, becomes another of the tools needed for work in authentication.

Basically, some series of coins have a consistent die axis, and others
do not.

When a new hoard of coins appears on the market with a consistent die
axis, but is of coins that should not, that suggests a problem.  The
same is true if the hoard is not consistent but is of a series that
normally is.

The trick is having enough known genuine specimens recorded within any
one series to determine if they should be consistent or not.

One should also remember that ancient coins are hand struck and "of
consistent die axis" really means + or - about 5 degrees with some
specimens fliped 180 degrees and the occassional specimen rotated a
greater amount.  No series is 100% ceonsistent, so a single specimen
cannot be condemned because it is not of correct die axis but it does
suggest a closer examination is in order.  A large number of specimens
togeather that are incorrect suggests a more serious problem.

Put this togeather with what I wrote yesterday about the flow lines
and you start to build a set of tools.  A coin with now visible flow
lines is highly suspect, but can be condemned on that alone.  A coin
with incorrect die axis is suspect but cannot be condemned on that
alone.  A coin with no flow lines and incorrect die axis is very
highly suspect, but even that is not enough to condemn it, but it
suggest even more things need to be examined (weight, style, look and
feel of the metal, engraving style, striking style, etc, etc) to be
sure.