possible Tyrian purple

[Diana Blumberg]

Hello everyone,

I am in the process of analysing a very strange purple material that turned up on an archaeological excavation near me. It is a waxy material, so I guessed it might be at least partially organic. I did a general XRF analysis on it with our Tracer III-SD (40kV, 10 μA, no filter), as well as using the red (40kV, 15 μA) and yellow (40kV, 10 μA) filters. If it is Tyrian purple, it should have bromine in it. Tyrian purple is also known as 6,6'-Dibromoindigo, or (BrC₆H₃C(O)CNH)₂

The problem I'm having is that the material shows a lot of lead, and I think arsenic and mercury, as well. All of this is hiding what I think is a small bromine K-alpha peak, underneath the As K-beta peak, which is slightly skewed to the right. I also see a small peak for Br L-alpha at 1.48KeV on the spectrum from general analysis, but I obviously don't want to rely on that. I'm using ARTAX to analyse the spectra I've collected. I've tried finding any relevant escape peaks and sum peaks, but I haven't yet considered matrix effects (would there be any?).

I have also done ATR-FTIR on the material, and the spectrum looks to me like a combination of Tyrian purple and beeswax. I'm basing the interpretation on several articles I've read, and on the database in our FTIR instrument for the waxy substance.

I will be doing another XRF analysis with our new Tracer 5g. 

I'm not interested in a quantitative analysis. I only want to know if bromine is present, even if in a small quantity. If it is present, then I have to explain why there is so much lead, and possibly arsenic and mercury in it (Pliny the elder does say that lead containers were used in the preparation of Tyrian purple). If bromine isn't present, then the material may be a more modern dye, such as aniline/mauvine.

Any suggestions for how I might tackle whether bromine is present would be greatly appreciated.

Kind regards

Diana Blumberg

Archaeology Lab Technician

Newcastle University

Newcastle-upon-Tyne, UK

[Tasos Asvestas]

Hello Diana,

Would it be easy for you to share the spectra you collected? We could give them a look and perhaps someone could assist more.

Kind regards

Tasos Asvestas

[Diana Blumberg]

Hi Tasos,

Yes, here are csv files for three of the spectra I collected: one for the general analysis, one with the red filter, and one with the yellow filter. 

Many thanks

Kind regards

Diana

[Tasos Asvestas]

Hi Diana,

I just checked your spectra and to my eyes the Bromine presence is almost certain, i cannot think of something else that could explain this "bump" on the right of the As Kβ but also and most important the presence of the Br Kβ peak! I attach a file (xls) of a fit i made with Pymca (choose any software you use to see how the element peaks are distributed).

I hope this helps you. 

Tasos

[Diana Blumberg]

Hi Tasos,

Thanks so much for taking the time to do this! Very helpful, indeed. If it ever comes to a publication, you will certainly get an acknowledgement! 

Now I have to explain why there are other heavy metals in the material.

Best regards

Diana

[Alexander Seyfarth Mineralogical Analysis]

Hi!

Check also the Al peak area for the Br L line.

Br Ka should be readily ID’ed even next to Pb LA using ARTAX.

Happy to review the file ..

Alexander

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<Purple_Pymca.xlsx><Screenshot 2024-03-27 143041.png><purple.png>

Alexander Seyfarth Mineralogical Analysis

miner...@gmail.com

Janesville USA