2 EXPERIMENTAL METHODS

TWO MODERN daguerreotype step tablets were used to compare the effects of cleaners for tarnish removal on daguerreotypes. These daguerreotypes were made using 19th century methods9 on daguerreotype plates manufactured in this laboratory, and the daguerreotypes were imaged with a seven-step step tablet which allowed relatively large areas of like exposure and apparent density to be compared.10 One daguerreotype was gilded, one was left ungilded. Because gilded daguerreotypes are more representative of 19th century daguerreotypes,11 the following discussion concentrates on the results obtained from the gilded daguerreotype. It should be noted that these daguerreotypes were several years old, but not appreciably tarnished.

The daguerreotypes were cut into five strips. For each daguerreotype, one strip was left untreated as a standard for comparison. One strip was placed in a 1% solution of potassium cyanide (KCN) for five minutes,12 one strip was placed in a “standard” solution of thiourea daguerreotype cleaning solution of 70 grams thiourea and 80 cc phosphoric acid per liter of solution for five minutes,4 and one was sputter cleaned in an O2 and Ar sputtering gas mixture for four minutes. The reactive rf sputtering conditions were: rf voltage (300 V); rf power (50 Watts); and a gas mixture of 10 mTorr Ar and 1 mTorr O2. The fifth strip was sputter cleaned for one minute using the same sputtering parameters as above.

In order to examine the gross effects of cleaning on the overall appearance of daguerreotypes, particularly with regard to “fading,” the total reflectance of both highlight and shadow regions of all samples was measured using a Beckman DK2a Ratio Recording Spectrophotometer (BaSO4 coated integrating sphere, BaSO4 standard, and PbS detector) with a tungsten source for wavelengths from 800 to 400 nm. Topographic comparisons were made using micrographs taken with an ISI Model DS130 Scanning Electron Microscope (SEM) and an ISI Model Super IIIA SEM. The microstructural effects were compared using a computer controlled JOELCO 50A SEM with Computer Evaluated Scanning Electron Microscopy Images (CESEMI) capability.13

In the CESEMI system, a PDP 11/20 computer (Digital Equipment Corp.) is interfaced with the JOELCO 50A SEM. This equipment design allows the computer to control the electron beam of the microscope as the beam is rastered across the sample surface. An energy dispersive x-ray detector (EDX) and a multichannel analyzer are used to monitor the signal coming from the sample surface. A particle is recognized when the x-ray signal coming from the sample surface is amplified above a certain preset background level. The computer then stops the raster motion of the beam and the beam is used to examine the particle. The particle size, shape, and average diameter are collected, as well as certain chemical information. After repeated evaluations of different areas over the sample surface, the computer gives the total particle count and a statistical analysis including average particle size distribution, particle shape distribution, and surface/volume percent (or how much of the surface is covered with particles). One of the major advantages of CESEMI is that the sample surface is monitored and evaluated directly and independently of visual display or recognition.