6.1 A substantial difficulty in using leather in applications requiring very long service lives is estimating how well a particular leather will actually hold up in service. Such applications may include use in musical instruments such as pipe organs, bookbinding leathers, etc. Use of leather in pipe organs in the past (prior to approximately 1930) demonstrated service lives frequently over 100 years, and the consequences of short service lives can result in extremely costly repairs. Many post-1930 leathers have had short service lives (as little as 15 years) due to the use of various more modern tannages and processing methods. Identifying exactly what tannage and processing was used in a particular leather and how successful this tanning and processing was can be very difficult. Failure of bookbinding and upholstery leathers formed the impetus for extensive work by leather chemists in the 1940s to identify tests that could be used to verify the durability of leather samples. Early studies by Cheshire3 and Frey & Beebe4 resulted in tests relating the rate of deterioration of leathers having known durabilities from long-term storage of samples, to the deterioration experienced by laboratory exposure of specimens to known contents of acid gases in air or oxygen. They were considered to be applicable to leathers having a wide range of tannages and processing. Later work published by Piltingsrud & Tancous5 described their modifications to those tests. Further work directed towards verifying the durability of leathers used in pipe organs resulted in the practice described in this document. The appropriateness of its use for any given leather samples must be determined by the leather chemists utilizing the practice. This practice may not be applicable for leathers having unusual tannages or treatments. Estimates of service lives made using this practice are speculative, as it would take many decades of natural aging to verify the results (see comments in Section 11).