Tuesday 29 October 2013

Analysing the Leutwitz Apollo (8): CMA's Conservator's take

In November 2003, that base plate was still on people's minds. Bruce Christman, CMA's conservator submitted a conservation report needed for acquisition. There is talk of the x-ray analysis and the ancient joins between various parts in the casting process (p. 60). None of this is properly presented in Bennett's "exhaustive" presentation. It is odd to find that although Northover's analytical results are not detailed, Christman's are (p. 60). The right foot is cast of a 10% tin-bronze with "between 10% and 15% lead". Bennett's main interest in Christman's work (pp 58-9) is in that base. He had carried out a close visual examination with careful attention to the areas of contact between the base and the figure. He concluded that the "base and sculpture shared a long history. Here Bennett notes (p. 58) that Christman records that only the right  foot has the chased position line. Dr Lie said two of them had (p. 56) casting some doubt of the reliability of the rest of his reported observations. Like Lie, Christman says that the corrosion products in the traced line "seem to be very old" (p. 58).
There is residue of lead-based solder under the feet and on the base in exactly the correct locations, sizes and shapes to accord to where the feet and tree must have been joined to the surface of the base. [4] There is no indication that the original join between the figure and the base had ever been disturbed since the figure was separated from it [5] a thin layer of green corrosion appears on top of the lead-based solder [...] [7] there is no  evidence whatsoever that either the base or the areas under the feet were ever artificially treated to make the joins appear old.
So, here's a question for Mr Christman. How did copper corrosion products get on top of the lead corrosion products in the joins if, when the statue was (allegedly) standing in the open, the lead is sandwiched between the statue and base plate? Inside the joint and inside the statue the lead will have been kept dry by the fact that its inside the statue, so how is it corroding? Surely this lead is only corroding after the statue has been separated from it - but then according to the CMA's collecting history it's on the ground floor of a house, indoors. Does lead corrode to the degree we see here indoors in a period of four decades? If the plate is lying on a floor, how do the copper corrosion products from underneath the lead migrate to its upper surface, through the lead? This is unexplained. Christman admits as much when he writes:
The corrosion observed on the lead-based solder and on the bronze base are consistent with a piece that has been in an outside environment or buried for several centuries [...] technically it is not possible to join heavily corroded pieces of metal together without cleaning the corrosion off by abrasion or using acid-based fluxes that would disturb the corrosion layers. Based on these observations, the area of the joins appears not to have been disturbed for at least a century and probably longer.
Now wait a second. Does that last bit mean that the copper corrosion products go under the lead? In the case of the base plate, why can the solder not have been applied to the patinated copper alloy plate using some kind of chemical flux and then the edges of the two areas obscured by inducing artificial corrosion of the lead (higher on the reactivity series than copper)?

Much is made of the lack of evidence of chemical patination of the lead (allegedly it would produce much more even corrosion products than Mr Christman observed). But then, lead is particularly susceptible (as any museum conservator will know) to organic acids. Stick lead tokens in a drawer with a hardboard bottom or attacked to some grades of cardboard and you are asking for trouble. Chinese fakers bury items like this in a goatshed. Next year they have a wonderful patina of age.

I do not see how Mr Christman sees this operating. If the corrosion on top of the sheared solder joins "looks old" then it got like that when the statue was not on it. In that case, how likely is it that this statue was standing on that plate in 1935 if the CMA want to show that this corrosion developed a hundred years ago? I think we need much more detail about what those solder joins look like, where this corrosion is and where it is not, and also what the chemical composition of that corrosion is.

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