Blue Thread of the Tzitzit: Was the Ancient Dye a Prussian Blue or Tyrian Purple?

The Radzin process for renewing the use of the Biblical blue thread on the Jewish ritual fringed garment (tz\Xz\xj is examined. The colouring matter formed proved to be Prussian blue. Since a natural dye obtained from a shellfish is required, the Radzin product is disqualified as an authentic revival. An alternative identification of the ancient blue with Tyrian purple is inconsistent with the accepted designation of the latter as the Biblical purple “argaman”. The Biblical blue is to be identified with the ancient hyacinthine purple, which was evidently derived from Murex trunculus. The Janthina hypothesis is also discussed.