Silver and Diazo

Microfilm Reel by Katherine Sleyko

Microfilm Reel by Katherine Sleyko; used under Attribution-NonCommercial-ShareAlike 2.0 Generic license

The State Archives of North Carolina provides microfilming of the minutes of major decision-making boards and commissions in state agencies and local governments. Once the minutes are filmed, we will store the “silver halide” original in our security vault. When we make a copy of the microfilm for state agencies or local governments to use in their offices, we make a “diazo” duplicate. You see both terms used in our duplication services fee schedule, but what exactly do they mean?

When minutes are microfilmed, a silver halide original microfilm reel is produced. According to the South Carolina Department of Archives and History, “Silver gelatin film, also called silver halide, has a base of either polyester, acetate, or triacetate coated with silver halide compounds held in a gelatin emulsion. When light passes through the film, it strikes and solidifies the emulsion to form a latent (invisible) image. To make the image visible, the film must be chemically processed, fixed, and then washed.” Silver gelatin film produces images of high quality and, importantly, its potential life expectancy “of up to 500 years makes it the only film suitable for filming permanent records.”

Diazo is another type of microfilm. According to the South Carolina Department of Archives and History, diazo film “has a triacetate or polyester base coated with an emulsion of diazonium salts, dye couplers, and a stabilizing agent. The film is exposed by ultraviolet light, which breaks down the diazo compound; it is developed and fixed by exposure to ammonia.” Diazo microfilm’s high resolution makes the quality of the image only about 4% less than the quality of the original. Additional copies can be made from diazo microfilm. Diazo costs less than silver, and, finally, “it can take more wear and tear.”

The last of these qualities makes diazo perfect for “access copies” of microfilm, i.e., microfilm for loading into readers and reading. For example, the State Archives’ Search Room has thousands of diazo reels of microfilm for the use of researchers. However, the microfilmed permanent records that we keep in a secure vault as human-readable preservation duplicates are longer lasting silver halide originals.

To learn more about the State Archives’ technical standards for microfilm and microfilm processing, please read our Micrographics Technical and Legal Procedures.