The single leaf of a carved ivory consular diptychs that will soon grace the Museo Arquelógico Nacional in Madrid following its purchase for £680,000 at Sotheby’s.

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But these pieces, known as consular diptychs, do have a bibliographic signficance. The diptychs came as hinged pairs that were lined with wax to form writing tablets.

In the Late Roman and early Byzantine period should you be awarded the high honour of being one of the two consuls elected annually to the two parts of the Empire (centred at Rome and Constantinople) you were issued with a diptych.

Prior to this period a scroll would have been used which makes these the earliest form of manuscript in codex format with hinged pages. As such they can be regarded as the oldest book bindings.

In AD 384 the emperor Theodosius enacted that precious ivory could only be used for these diptychs (diptycha ex ebore) and no others. It was once supposed that they were used to enshrine the official instructions of consular office. However, from at least 487, the figure of the consul is shown, as here, holding a mappa, the rolled cloth used as a starting signal for Roman races, and it has been suggested that such diptychs were presented to consuls at their inauguration and used during the public reading of the list of competitors and performers at the celebrations.

The offices of Consul were abolished in Rome in 534 and, finally, by Justinian in Constantinople in 541.

Sotheby's put forward a fascinating if hypothetical early provenance for this particular diptych as the property of Boethius and/or of Cassiodorus who were consuls in 510 and 514 respectively. Both these men were major figures in early Christian history who wrote books which are still occasionally read today.

A further hypothesis is that the diptych could have been owned by Benedict Biscop who brought some of Cassiodorus' manuscripts back to Northumbria where they could have passed through the hands of the Venerable Bede (d.735).

This makes the interesting point that not all antiquities are necessarily excavated - some have remained above ground since the day that they were made.

As to its more recent provenance, Sotheby's ivory was in the Barberini Library until the end of the 19th century and may have entered the Barberini collections through Cardinal Francesco Barberini (1597-1679) who had an extraordinary medieval art collection. By 1909 it was in the collection of the Comtesse de Béhague and featured in the Béhague sale in Sotheby's Monaco rooms in 1987 when it was sold to a French private collector for the then French Franc equivalent of £180,000.

These are rare pieces. Only 41 are known to have survived and (with the exception of two recorded in the 19th century whose whereabouts are now unknown) this was the only one known to be in private hands. Now this diptych leaf is also in the amiable captivity of an institution having been bought at Sotheby's sale for £680,000 (plus premium) by the Spanish Ministry of Culture.

It may safely be assumed that it will soon grace the Museo Arquelógico Nacional in Madrid. This, I may add, is one of the most magnificent museums anywhere and is not as celebrated as it should be. Go there!