Penetrating Ink
Ink that contains a penetrating dye that goes into the fibers of the print-receiving material and shows through to the back of the document. It is usually used in passports and travel documents for applying numbers by letterpress printing.
a
b
c
d
e
Fig. 1. Hungary. Passport issued in 2002:
a — page spread (page 32 – data page). Paper substrate (page 32) and polymer substrate (data page); b — the same. Blank number. Front side. Penetrating ink; c — the same. Document number. Back side. Traces of red dye on the reverse side of the print. Oblique light; d, e — the same. Zoomed fragment. Front (d) and back (e) side of the print. Letterpress
a
b
c
Fig. 2. Slovenia. Travel document (Convention of 28 July 1951) issued in 2016:
a — page spread (front endpaper – page 1). Paper substrate; b, c — the same. Blank number (b). Zoomed fragment of the symbol (c). Penetrating ink. Letterpress