As placed on the BCHF forum and discussed with the SART team and a few other spots.

And Tischendorf coloured the sheets that came out in 1859. This led to the amazing situation where you can actually see and compare the BEFORE 1844 white parchment that went to Leipzig and the AFTER 1859 yellowed, streaky, stained parchment now in Britain. Rarely does such a crime leave such an easy to see and follow trail.

Plus, the crime of colouring the manuscript to make it look old was specifically noted as having occurred in the 1850s. This was noted by an observer who was at the monastery, Kallinikos, and published in 1862-64. Kallinikos also noted the 1844 theft. And that the "loan" of 1859 was a cover for theft and would not be returned.
This can also be noted in a chronological fashion.

1844 - Tischendorf takes 43 folia of white, unstained parchment to Leipzig (same colour today) "phenomenally good condition"

1859 - Tischendorf takes about 350 folia of yellowed, strain and streaky parchment to St. Petersburg, "phenomenally good condition"

1862-1864 - colouring by Tischendorf of the manuscript in the 1850s, is stated by Kallinikos who witnessed it at St. Catherine's, matching perfectly what we see today


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Why is it amazing?

We can see the BEFORE and AFTER very easily with our own eyes, due to the professional photography of the CSP!

A good start is http://sinaiticus.net/

And the vlogs at:
http://sinaiticus.net/video%20resources.html
Who Darkened Siniatics
Colouring the Truth


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Some additional notes.

1862 - Tischendorf creates a fascimile edition that hides the difference, and says little about parchment colour

1860-2009 - the two sections are kept far apart and with difficult access

2009 - Codex Sinaiticus Project and internet studies begin to expose the shenanigans