Authenticating Old Instruments

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Article taken from "Backsights" Magazine published by Surveyors Historical Society

FIELD WEAR:  Authenticating Old Instruments

by David St. John

David St. John, who has examined hundreds of old surveying instruments in his long career, discusses what to look for when evaluating a pieceís authenticity.

Early instrument makers often formed lasting relationships - either with respect for, or in contempt of, one another - and would share or copy methods, material selections, and design principles. Each sought to produce more accurate instruments and capture a larger market with innovations that overcame various technical problems.

One axiom for this period, accepted within the instrument industry, is the following: An instrument maker always leaves his mark. This is especially true when studying fabrication techniques. The production of threads, knurls, chamfers, tapers, screw heads, bezels, cast metal, glass (both float and optical), engine divided circles and verniers, soldered or inlaid sterling silver, stamped or engraved numbers, uniform depth and width in graduations, and compass face embellishments is unique to each craftsman or shop.

The philosophy An instrument maker always leaves his mark is useful in the determination of provenance, and helpful when resolving questions related to whether an early instrument is in fact the work of the original maker, or a premeditated attempt to produce a forgery. Unfortunately, such a determination often requires disassembly of the artifact and then the application of metal testing techniques. This type of analysis should always be performed by a professional skilled in the art of preservation and restoration. However, there are certain visual tests of general appearance which can be performed by the layman to make an initial determination concerning an itemís authenticity.

Mechanical Component Wear

There are few exceptions to obvious wear on early scientific artifacts, even if such instruments have been properly used and well maintained in a laboratory environment. Even later instruments, in appropriately conserved collections, have obvious evidences of wear peculiar to their function. This evidence affects leveling screws, footplates, objective slides, eyepiece assembly to rear bearings, rack & pinions, telescope axes, tangent screws, and capstan head screws.

Such wear is measurable, but can usually be detected by a visual and tactile examination alone. Normal wear is peculiar to functional surfaces. An example is the leveling, or tangent, screw. In general such components are used near the center of their range of motion, and the resulting wear-pattern falls predominantly in the center of their travel. Such wear is relatively consistent whether or not the instrument has been well-maintained while in service. This wear is also accelerated when an instrument has been used in a hostile environment such as a desert with blowing abrasive particulate, or in a mining, tropical or coastal operation where corrosion can impact functioning surfaces.

Handling Wear

This occurs particularly on thumb screws, focussing knobs, leveling screw knobs, and lock screw knobs. This type of wear has a peculiar burnished effect caused by the oils and acid of the fingers. Knurls can be worn partially smooth on much-used knobs. In certain cases, the face of knobs where human contamination was left for long periods of time will reveal the grain of the metal where the surface etched through the protective finish. This type of wear can also be found on fixed surfaces adjacent to a knob, such as the contaminant immediately around a focusing knob on a telescope, or at the periphery of the compass plate near the upper tangent clamp.

Ambient or Environmental Wear

The best example of environmentally-caused wear occurs when wind-driven particulate impacts protective finishes and causes pitting corrosion, which with time and proper conditions becomes area corrosion. The optical glass will also have damage from the impact of abrasive particulate, in addition to damage created by any operator attempting to clean the objective and eyepiece surfaces. The initial impact will cause microscopic pitting, while the attempt at cleaning will produce fine scratches with circular form caused by dragging abrasive particles over the surface.


This type of wear falls into the environmental wear classification, and it is caused by rain, or condensation, both external and internal. Even carefully maintained instruments were typically used in the cold and then brought into warm environments where the humidity would condense immediately on all surfaces. The internal condensate could not be conveniently removed and would eventually initiate corrosion on active surfaces including threads. This type of evidence is commonly seen between the elements of microscope objectives and is destructive to both glass and metal.

Once this type of corrosion is initiated the Galvanic couple resulting from the active surfaces of different metals being in contact with each other would permit a self-sustaining corrosion site. Silver having an E.M.F. of 0.0 volts coupled with brass having an E.M.F. of -0.35, or lead with an E.M.F. of -0.50 volts can have corrosion which is particularly aggressive in instruments when stored in uncontrolled environments, particularly while in service. The conservator must be aware of this type of corrosion and be prepared to neutralize the problem, remove corrosion products, and further protect the active surfaces with an appropriate treatment to isolate the active surface from the environment, or store the artifact in a desiccated environment held below 30% R.H. at all times.

Fungus Growth

Whether active or dormant, fungus growth is promoted by storing materials in dark, moist, and warm environments. A typical example is an instrument stored in its carrying case in an office or closet for long periods of time. Fungus is particularly damaging to glass surfaces. The substrate is not a nutrient, but the cilia of the fungus grows rapidly and its excretion dissolves out the soda in early glasses. The fungus may be treated and the glass cleaned, but the pattern of the fungus will be permanently etched into the glass surface. To a viewer, this will diffuse image-forming light into a soft haze. Fungus will also aggressively attack organic or metallic materials and must be treated in order to save delicate components, especially spun-in fins and thin sections of optical assemblies.

Color of Optical Glass

In glass-making for scientific instruments, quality depended upon the formula used, the purity of materials, and the type of production. Early methods created imperfect float and optical glass, resulting in air bubbles, striae, and defects of color. By 1883, the combined investigations of Abbe and Schott produced an optical glass exhibiting uniformity in property and color. Evidence of glass imperfections will help date an instrument.

Oxidized Lubricant

Early instrument makers reduced the friction of moving parts by using lubricants of animal origin. These lubricants congeal with loss of vehicle and collect contamination from adjacent components as they deteriorate. No longer acting as lubricants, these early materials begin to conduct E.M.F. and the resulting corrosion eventually causes functional components to seize. Lubricants in this condition generally have color related to the salts of the materials involved.

The above initial evidence has nothing to do with provenance; it relates to age, application, general ambient environment, maintenance, and perhaps the general geographic location where the instrument was used during its active life. At least a few, if not all of the above conditions can be expected to exist to some extent on a scientific artifact. A forger would understand that a keen eye would be looking for this type of evidence and would artificially simulate the characteristics anticipated. Fortunately, the physical, optical and chemical characteristics of artificial aging are different from those found resulting from an intimate exposure to environment and time. Thus, a carefully examined scientific instrument will yield up many clues pertaining to its life in the field.

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