Method of currency or document validation by use of a temperatur

Details Method of currency or document validation by use of a temperatur Method of currency or document validation by use of a temperatur

1994-09-13

1996-07-09

Bryant, David P.

Printed matter

Method

Identifying

283 57, 283 82, B42D 1500

Patent

active

055337597

ABSTRACT:
A low Curie temperature magnetic material (such as chromium dioxide) is used as a magnetic pigment to validate valuable documents such as banknotes. CrO.sub.2 is very black in color and is an excellent magnetic recording medium, and has a Curie temperature of 128 degrees C. A region of a banknote or other valuable document is printed with an ink containing CrO.sub.2 particles. To test the validity of the document, the magnetic media on the document is subjected to magnetic field having a characteristic spatial pattern; the field of a permanent magnet having alternating magnetic poles is a convenient field source. The banknote, and its magnetized region, is then brought to a temperature of at least 128 degrees C, which is readily accomplished by use of a heat lamp, and the region inspected with a magnetic field sensitive optical reader. If it is a genuine bill whose magnetized region was printed with an ink containing CrO.sub.2, the recorded magnetic pattern will have disappeared as the media becomes non-magnetic above its Curie temperature. A counterfeit, if recorded with an ink containing Fe.sub.3 O.sub.4, will retain the recorded pattern when heated to 128 degrees C, as its Curie temperature is about 585 degrees C.

REFERENCES:
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patent: 4396886 (1983-08-01), Koester et al.
patent: 4438462 (1984-03-01), Koester et al.
patent: 4455484 (1984-06-01), Whitehead
patent: 4584529 (1986-04-01), Aoyama
patent: 5190318 (1993-03-01), Mantegazza

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