Yitzhak HaLevi Herzog (born 3 December 1888 – died 25 July 1959), also known as Isaac Herzog, was the first Chief Rabbi of Ireland, his term lasting from 1921 to 1936. From 1937 until his death in 1959, he was Ashkenazi Chief Rabbi of the British Mandate of Palestine and of Israel after its independence in 1948.
Rabbi Herzog was born in Łomża, Poland, and moved to the United Kingdom with his family in 1898, where they settled in Leeds. His initial schooling was largely at the instruction of his father, Joel Leib Herzog, who was a rabbi in Leeds and then later in Paris.
After mastering Talmudic studies at a young age, Yitzhak went on to attend the Sorbonne and then later the University of London, where he received his doctorate. His thesis, which made him famous in the Jewish world, concerned his claim of re-discovering Techelet, the type of blue dye once used for the making of Tzitzit.
Rabbi Herzog served as rabbi of Belfast from 1916 to 1919 and was appointed rabbi of Dublin in 1919. He was Chief Rabbi to what (from 1922) became the Irish Free State. He was known as “the Sinn Féin Rabbi” He went on to serve as Chief Rabbi of Ireland between 1922 and 1936, when he immigrated to Palestine to succeed Rabbi Abraham Isaac Kook as Ashkenazi Chief Rabbi upon his death.
Rabbi Yitzhak HaLevi Herzog obtained a sample of the Radziner dye and had it chemically analyzed. The chemists concluded that it was a well-known synthetic dye “Prussian blue” made by reacting iron sulfate with an organic material. In this case, the cuttlefish only supplied the organic material which could have as easily been supplied from a vast array of organic sources (e.g., ox blood). Rav Herzog thus rejected the cuttlefish as the chilazon.
Within his doctoral research on the subject tekhelet, Herzog placed great hopes on demonstrating that the Murex trunculus was the genuine snail chilazon. However, having failed to achieve blue dye from the Murex trunculus, he wrote: “If for the present all hope is to be abandoned of rediscovering the chilazon shel techelet in some species of the genera Murex and Purpura we could do worse than suggest the Janthina as a not improbable identification”. Although blue dye has indeed been obtained from the Murex trunculus snail, in 2002 Dr. S. W. Kaplan of Rehovot, Israel, proclaimed that he was able to dye wool with the extract of Janthina. This claim has to date not been substantiated.