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Blue dag 1
(By courtesy of Paul I.Johnston)

Blue dag 2
(By courtesy of Paul I.Johnston)

This sounds like the title of a thriller, doesn't it?

The purpose of this page is to resolve the strange case of those daguerreotypes appearing with a heavy dominance of blue all over their surface.

Most daguerreotypes present a sharp focus which reveals a high level of detail, even after more than 150 years of existence. So, the case of dags showing such a saturation of blue color instead of the regular grey levels they normally offer, is quite strange.

Different assumptions have been evoked to explain this curious and uncommon rendition of the image:


The first one is based on the possible oxidation of the plate. As any piece of silver, daguerreotype plates become tarnished when exposed to the atmospheric pollutions. This deterioration is induced by sulfide and silver oxide covering the whole plate in a colored film. This can appear as yellow, red or blue depending on its thickness. Generally, this deterioration is progressive; starting from the middle of the plate and growing progressively to its edges.

This is clearly not a valid explanation for a true "blue dag" which is evenly blue and was like this from its first day. It is possible to find blue dags for which the photographer had to tint clothes such as shirts, to make them more visible.


The other option which is also put forward is the possibility of a too intense warming of the plate during the gold toning process.

This explanation is not relevant here, as blue dags exist which did not receive a gold toning process. More, in case of an inappropriate warming, the blue color would not appear homogeneous on every part of the plate, which should have taken different temperatures.

In fact, the true explanation is just a too intense solarization (overexposure) of the plate during the shooting of the portrait.

The image on the right gives evidence of this issue. The white areas of the image appear irridescent, as if the metal was visible.

Today, an e-photographer would just erase the over-exposed image from his digital camera, but in the 1840s, a portrait was something not so common, long to obtain and expensive. In such conditions, even when not so perfect, clients kept these blue portraits with them for posterity.

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