Priestley Burning Glass 

Dickinson College


Joseph PriestleyThe Priestley Burning Glass was a scientific instrument designed and utilized by Joseph Priestley to test his theories on the relationship between oxygen and the combustibility of certain materials. The apparatus used a large wooden stand supporting two glass lenses, which Priestley used to focus the suns rays on a material located within an enclosed, airtight bell jar. He would also utilize an air pump, which he would attach to the bell jar to either introduce or remove material. Priestley was a key figure in the discovery of oxygen and the requirement of oxygen to be present in order for combustion to occur.


Joseph Priestley (March 13, 1733 - February 6, 1804) was a gifted scientist and well known in the English scientific community. He discovered several gases and invented soda water. He was the author of several scientific and philosophical papers wherein he defended the phlogiston theory, the idea that all flammable materials contain phlogiston, a substance that is liberated in burning. It was within his efforts of proving this theory that he used his burning glass apparatus. Additionally, he was a theologian and philosopher, which caused great turmoil his life because many of his ideas dissented from the beliefs of the Church of England. Priestley attempted to combine theism, materialism, and determinism with Christian theism. Persecution due to those beliefs eventually caused him and his family to flee Europe and settle in the Northumberland area of Pennsylvania in the mid 1790s.This is where he remained until his death in 1804.



Before Treatment: Split WoodBefore his death, Priestly donated his entire library and much of his equipment to the Northumberland Academy. Some of these materials, including the burning glass, eventually found their way to Dickinson College, located in Carlisle, PA. The condition of the burning glass was extremely fragile. It had several splits and breaks in the circular wooden lens surrounds. The larger upper lens was damaged in the form of numerous chips along its perimeter edge. The object had also been restored several times as evidenced by numerous inserts. These inserts caused additional fractures by not allowing sufficient space for the expansion and contraction associated with changes in relative humidity. Additionally, old fills are darker than the surrounding areas and there are deep sanding scratches where many of these areas have been repaired or restored. Because of the condition of the burning glass, B.R. Howard & Associates, Inc. were contracted to conserve the piece and prepare it for public display.



During Treatment: Prepping for ToningAfter transporting the burning glass to the studios of B.R. Howard & Associates, the apparatus was digitally photographed and its condition extensively documented. After completing said documentation, the instrument was disassembled and the lenses removed from their wooden bezels. A fracture on the upper edge of the pedestal was then repaired using hot hide glue. All interior surfaces of the pedestal were then coated with a isolation layer of reversible varnish. The pedestal itself was then modified with a polycarbonate tube to more securely hold the brass shaft upon which the optics rotated. Discolored wooden components were re-toned and areas lacking finish due to previous repairs or restorations were toned and finished with specially formulated shellacs. Areas of disfiguring scratches were reduced and toned appropriately. Brass components also had deep scratches which were reduced using consecutively finer grit silicon carbide papers. The brass was then polished to a high sheen using jewelers rouge on a cloth wheel. The larger focal lens' chipping was consolidated using a specially formulated clear epoxy and then repositioned into the wooden bezel. Remaining wood components were repaired as required. Every component was then coated with varying protective lacquers as per material requirement. The apparatus was then reassembled and mounted onto an appropriate base using the same screw holes as previously utilized. Upon completion, the burning glass was returned to Dickinson College's Special Collection.







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