Melina Avery first encountered ozalids during a fellowship at Northwestern University Library when a large collection of music manuscripts and reproductions arrived in the Conservation Lab. Avery reports that “Ozalid” is the patented name of a diazotype reproduction process, but in common usage in music libraries, the term “ozalid” can also refer to photostats, mimeographs, blueprints, and other pre-Xerox reproductions.
Many original music scores have never been published, and delicate originals (often written on thin, “onion-skin” paper) were frequently discarded once reproductions were made. As a result, ozalids may be rare or unique copies of a given music score. Because so many different processes have been used to reproduce music manuscripts over the years, it can be challenging to firmly identify the process used and determine best practices for treatment and housing.
Avery surveyed the ozalids in Northwestern’s collections and, through visual identification, determined that 37% were diazotypes. The diazotype process was invented in Germany in 1923 and involves a reaction of light-sensitive chemicals with ammonia to produce a blue, maroon, brown, or black image. Because the chemicals were not rinsed from the paper in this process, diazotypes tend to display distinct patterns of deterioration, including darkening or discoloration of the image-side of the paper and loss of image contrast.
Avery was fortunate to acquire samples of known types of ozalids from a local music publisher to use for further testing in order to establish treatment protocols. She focused her research on diazotypes, which were the most common type of ozalid held in Northwestern’s collections.
Despite visual identification, Avery hoped to develop objective tools for identification using FTIR. She analyzed the front and back of the ozalids, and compared results to known diazotypes. Unfortunately, the spectra gave only ambiguous results.
Avery subjected ozalid and diazotype samples to common treatments, including surface cleaning, humidification, mending, and tape removal with solvents. Although diazotypes can be sensitive to moisture and displayed feathering of the media on exposure to water, she found that humidification for up to one hour could safely be carried out. She does not recommend extended humidification due to the potential for feathering, bleeding, and sinking of the media. Diazotypes have also been reported to be sensitive to heat, but Avery’s test showed no color change when briefly heated with a tacking iron, as for mending with heat-set tissue. Ethanol and acetone both resulted in feathering or bleeding of the media, but toluene did not. Based on these tests, Avery concludes that many basic treatments can be undertaken to stabilize fragile ozalid collections.