One of the most stunning works of art on the Scripps College campus is the mural Flower Vendors(1946). The artist,Alfredo Ramos Martínez, an influential figure in Mexican Modernism, executed the work using a variety of traditional and non-traditional techniques. Prior analysis of the work indicated the use of a wax emulsion medium and established the range of pigments used. Ramos Martínez’s use of lead chromate (chrome yellow, Pb(CrO4)) was unusual in wall painting, and the pigment itself has been shown to photodegrade and darken over time in oil paintings, due to the reduction of Cr(VI) to Cr(III). The Pb(CrO4) in Flower Vendorsdoes not appear to have darkened, raising questions about the stability of lead chromate in a wax-emulsion medium relative to that of oil-based mediums. To better understand the behavior of lead chromate in wax-based mediums, a historical synthesis of lead chromate was recreated, and the pigment was suspended in four different binder matrices: a wax-water emulsion, refined linseed oil, cold-pressed linseed oil, and poppy oil. Each of these paint-binder mixtures wasaged beneath full-spectrum 6500 K LED lights. The relative darkening of the pigments was measured using UV-Vis reflectance colorimetry, and comparisons were made between the mediums.

This study was directed at the pyrotechnic time delay compositions that are used in detonator assemblies. The objectives were to: --Investigate effective alternatives for the barium and lead-based oxidants currently used, maintaining the use of silicon as fuel --Develop easy to use, realistic measurement techniques for burn rates and shock tube ignitability --Determine the variables that affect burn rate, and --Evaluate alternative processing routes to facilitate intimate mixing of the component powders. Lead chromate and copper antimonite were found to be suitable oxidants for silicon in time delay compositions. They were ignitable by shock tubing, a relatively weak ignition source. The measured burn speeds for these systems showed a bimodal dependence on stoichiometry. Measured burn rates varied between 6-28 mm/s. Lead chromate is potentially a suitable alternative to the oxidant currently used in the medium burn rate commercial composition. It burns faster than copper antimonite. The latter is potentially a suitable replacement oxidant for the slow and medium compositions. Antimony trioxide-based compositions exhibited unreliable performance with respect to ignition with shock tubing. The addition of aluminium powder or fumed silica was found to reduce the burn rate. Increasing the silicon particle size (<3,5<font face="symbol">m</font>m) also decreased the burn speed for copper antimonite and lead chromate compositions. Addition of fumed silica improved the flow properties of the lead chromate, copper antimonite and antimony trioxide powders allowing for easier mixing. The silicon powder was found to react violently with water in alkaline solutions. This makes particle dispersion in a wet-mixing process problematic. / Dissertation (MEng (Chemical Engineering))--University of Pretoria, 2006. / Chemical Engineering / unrestricted

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