Creosote has been the most widely used preservative for wooden railroad crossties or sleepers for over 100 years. Alternative preservatives such as copper naphthenate (CuN) have been shown to provide equivalent performance against decay fungi and wood destroying insects; however, the use and specification of CuN involves an education process for treaters and railroad customers alike. One difference between the two preservatives is use of a diluent or carrier oil with CuN, where both copper and oil retention become treating factors. Another is the ability to easily determine copper concentration in various zones throughout the interior of ties treated with CuN. Assay retention and other results-based specifications may be somewhat new concepts to tie treaters accustomed to measuring gauge retention based solely on volumetric or gravimetric uptake of creosote or creosote solutions. The objective of this study is to compare preservative retention in oak/hickory and mixed hardwood crossties treated with CuN in diesel oil solutions when measured by gauge and assay, and to determine if minimum copper retentions are achieved even when gauge retention (net solution uptake) appears deficient. Gauge and assay retention was measured from 272 commercial hardwood tie charges treated with CuN solutions, and copper retention was determined in core borings at various assay zones. The data confirm that adequate copper retention is achieved in both oak and mixed hardwood charges even when at relatively low solution retention levels, either because of refractory species or low copper concentration in the treating solution. Copper retentions measured by assay exceeded by 26-67% the values for gauge retention estimated from solution uptake, in part due to selective adsorption of copper.

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