Saturday, September 16, 2017

Black Sea Hoard dies

The American Numismatic Society hosted a seminar on Counterfeits: the Threat to Collecting and Scholarship with presentations by David Hendin, Robert Hoge, and Ute Wartenberg.

For me the most exciting part was inspecting dies from the group of counterfeits known as the “Black Sea Hoard.” In his book Counterfeit Studios and Their Coins Ilya Prokopov describes this hoard as the work of “Studio ‘Varna-1’”. The dies are said to be the work of a single master engraver. Coins produced by these dies were sold openly in museums as replicas. They were also being sold as genuine to collectors.

These dies represent the mints of Apollonia Pontika, Mesembria, Istros, and Chersonesos. The Chersonesos dies troubled me because they were unfamiliar. I recognize the style of the Varna-1 engraver's work on facing head coins. The Chersonesos dies are by the same sculptor. My inability to recognize them proves my understanding of that style of the artist's facing head fakes is overconfidence. I am not sure I would recognize fake Chersonesos coins without carefully consulting a scarce pamphlet-book Modern Forgeries of Greek and Roman Coins by Dimitrov, Prokopov, and Kolev.

Tuesday, August 29, 2017

The difficulty of photographing gold coins

Here are six professional pictures of the same coin.

These pictures appeared on the auction catalog or web sites of numismatic auctioneers Harlan J Berk, Classical Numismatic Group (twice), Heritage, Stack's Bowers, and the coin grading company Numismatic Guaranty Corporation.

These images were taken over the span of eighteen years. The version with the red background was scanned from a printed auction catalog. All of the other images are taken directly from auction sites or the slab company's slab verification image.

This coin did not change color in the last 18 years. Something about the lighting situation and the camera's color profile was different enough that each of these pictures is different.

What color is the coin really? In my memory the coin looks like the middle coin on the left-hand side. Yet when I held the coin over my computer monitor in a room lit by compact florescent lights it looked most like the upper right image.

Can the output of my computer be trusted? In the days of CRT monitors the color balance could be tuned and there were reference images for that purpose. LCD monitors are not as color-accurate as tuned CRT monitors. Perhaps I should repeat the experiment in the sunlight? Bottom-line: I am not certain which image is the most accurate.

Monday, July 17, 2017

Database of Digits Extracted from (modern) Coins

Xingyu Pan and imageLaure Tougne have published a paper describing their database of digits extracted from scans of modern coins.

The database of 3000+ images itself cannot be directly downloaded. They explain here that you may request it from them.

The paper is good! The databases discussed are

  • 606 digit images manually cropped from high-resolution photographs of well-conserved coins
  • 1,200 digit images automatically extracted from a subset of a PCGS database containing coins in different states of preservation
  • 1,200 digit images manually extracted from the same coin photographs as above

Although the new dataset only contains digits from modern coins the authors discuss the literature of recognition of ancient coin lettering. There are as yet no public databases of images of the digit or letters extracted from images of ancient coins.

Saturday, July 08, 2017

Antiquities Memorandum of Understanding with Libya

Please read Cultural Property Observer and comment on the Libyan MOU. If the MOU is completed and includes coins it will become difficult for Americans to buy ancient coins from Kyrene in European auctions. It will also be difficult to import Ottoman coins from the territory of modern Libya.

Do it this weekend or it will be too late.

Thursday, July 06, 2017

Recognizing Roman coins with neural networks

Jongpil Kim of Rutgers university reports a neural network capable of correctly recognizing the emperor 73% of the time from coin images. The reverse type is recognized 67% of the time.

The report is available on SEQAM and the paper is available by clicking PDF on this Cornell library site.

Wednesday, April 26, 2017

Tuesday, March 07, 2017

Video on creating 'coins' with CNC machine

I stumbled across this 9 minute video which shows the process of creating a medallion with realistic bust using a CNC machine.

The medallion produced looks quite good! It looks better than most of the 'coins' featured in do-it-yourself coin making videos. Yet most of the steps made me cringe because I was certain he was doing it the wrong way.

Once he has a 3D model on the computer, he feeds a block of metal to a CNC machine rated only for wood. I was sure something would go wrong but he got it perfect.

To produce the round shape he just takes his metal block and starts hitting it with a belt sander! I was certain he was doomed but it looked very nice when complete.

To give the appearance of patina he uses spray paint. I was amazed that this worked at all! He got a nice effect in the end.

The final medallion suffered a bit of a 'staircase effect' on the face due to the limited depth control. I would have liked to see if he could have removed that with a bit of polishing. Even with the depth artifacts he managed to produce a pretty nice medallion using techniques I would not have expected to be effective.