Bulletin 21, 1973

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The Compositional Analysis of 
French-Canadian Church Silver

by R. M. Myers and J. F. Hanlan
Canadian Conservation Institute

Pages  1  |  2  |  3  |  4

The same question of authenticity is raised by the chrismatory (accession number 16860.1 .2 .3 .4) by Amiot. The box had Amiot's mark on it, and it read a not-too-unreasonable 91.5%. However, the three containers inside were unmarked, and gave the lowest readings of any silver analyzed (as low as 85.1%, which is up to 9.9% lower than the box).

At this point it seems unlikely that chemical analysis alone will prove a point of authenticity of Canadian silver in an absolute manner. However, it will be an important method of adding to existing evidence.

The results of our study of this relatively limited collection are sufficiently encouraging, and, we believe, of sufficient significance to one aspect of Canadian history, that an extension of the project to include a study of works of other silversmiths from different areas of Canada is planned. It is hoped that enough pieces of domestic silver will be available for analysis to enable a meaningful comparison to be made with Church silver in general. As well, work has begun in the extremely interesting, but distinct, area of silver made for trade with the Indians.

Appendix

The electronic signal processing used in this laboratory is somewhat different than the usual pulse-height analyzer instrumentation commonly employed, and should be discussed briefly. A "black box" schematic is given in figure 7.

Pulses from the preamplifier are shaped by the amplifier, which is equipped with pole zero cancellation and base-line restoration. Coarse and fine gain controls can be pre-adjusted to any of four positions so that specific energies, i.e., pulse heights, can be set to fall into the desired channels of the encoder for full-scale values between 10-100 kiloelectron volts (KeV). A fifth position is variable over the entire range and can be used for unusual problems or preliminary testing.

The heart of the system is the encoder. For digital counts data, each pulse is converted to a digital number which is sorted into eight (in two sets of four) pre-selected "windows." The channel number and width are selected by program cards which are plugged into the encoder. These cards can also be programmed to produce marker bars for the channel selected. There are 128 channels available. For spectrum display, each pulse is used to drive the X axis of a storage oscilloscope. At the appropriate position, a line is written whose intensity is modulated to zero in the Y axis. As identical events occur, the trace on the oscilloscope screen intensifies into a vertical wedge. Visually, this presentation is equivalent to several thousand channels of a multi-channel analyser. The pattern on the oscilloscope is recorded by a camera equipped with a projected graticule accessory. A 15 KeV and 30 KeV graticule have been prepared and fine gain and position controls on the encoder permit accurate placement of the spectrum relative to the energy scale.

Digital data from the encoder are accumulated for a pre-set time (or pre-set count) by the set of buffered scalers. A scanner controlled by the timer and sequence controller reads the data in the scalers, and a character generator writes this information on the oscilloscope screen for up to four elements, along with the live-time of the analysis. This can be repeated for a second set of four elements programmed on the other card in the encoder, so that a total of eight elements can be determined per experiment.

The radio-isotope used for excitation is I125 adsorbed on charcoal beads and arranged symmetrically in an annular configuration. The source spectrum is Te Kx and Te Kb at 27.5 and 31 KeV respectively with a 'Y at 35 Kev and has a half-life of about 60 days. This inconveniently short half-life is largely compensated for by the fact that the material is cheap, very pure, can be obtained in high specific activities (100-300 mCi initially), and has a convenient energy. The main Compton peak is near Kx energy of Sn, and for this reason, the ring source is housed in a conical collimator fabricated from Sn so that the small amount of fluorescence from the collimator does not cause interferences in the remainder of the energy region. The source collimator assembly is about 7/8" thick x 2" in diameter. The high initial activity permits a useful source life of several half-lives of I125. The collimator can be closed down to irradiate a spot about 3 mm in diameter at a distance of 5 mm, but is usually used with a spot size of I cm.

Our operating procedures are extremely simple. The spectrum display is checked with a calibration standard of 80% Cr2O3, 20% SrO, and I% Ag2O (all percentages approximate). If the positions are slightly off; it is frequently faster to check an uncertain assignment by briefly presenting the standard for that element than to set the controls precisely. Alternatively, one can program in the channel marker for that element. The object to be studied is positioned so that the area(s) of interest are centrally located relative to the source and detector, at a distance of about 5 mm from the front of the collimator. The spectrum is then accumulated for a period which may be as short as one minute, for the major elements in a painting or sculpture, or as long as ten minutes for trace elements in a mineral or ceramic, or for detection of the small amounts of pigment in a drawing or watercolour. When a series of similar objects or a number of areas on the same object are studied, a uniform accumulation time is used in order to facilitate comparison. The scaler has typically been used as a simple clock, or to obtain the backscatter integral, or in the external clock mode to obtain the ratio of two peaks. The data is collected and stored on Polaroid photographs of the oscilloscope screen.

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