The influence of different tempers on the composition of pottery
The modification of the raw clay by the potter to produce a paste suitable for the intended purposes adds a layer of obfuscation to the problem of provenancing the original clay source by chemical fingerprinting. By preparing different pastes from the same commercially available raw clay and their chemical analysis by Instrumental Neutron Activation Analysis, an experiment under controlled conditions (tempers, mixing ratios, firing temperatures and sampling methods) sheds light on the influence of different tempers. The results show that two different sampling procedures (drilling and grinding) have almost no influence on the chemical fingerprint with the exception of the elemental concentrations of As, Zr, and Hf. This may be due to the volatility of the compounds (As) or the presence of zircon crystals (containing Zr and Hf) which are partly lost during drilling. Three different firing temperatures show no significant influence as well. The application of the modified Mahalanobis distance introduced by Beier and Mommsen in 1995 as a statistical filter and the introduction of a ‘dilution factor’ to the raw data show that the influence of quartz-dominated tempers can be filtered out of the data, resolving the underlying chemical fingerprint of the original clay source. At the same time, by mathematically removing the additional spread introduced by dilution, even subtle differences between similar pastes can be resolved by standard multivariate statistical means.
A Petrographic Analysis of Extended Middle Missouri Ceramics from North Dakota
Abstract This article summarizes a petrographic analysis of ten thin sections prepared from Riggs and Fort Yates ware sherds (ca. A.D. 1200 to 1450), the first such study ever conducted on prehistoric ceramics collected in North Dakota. The procedure identified, described, and estimated the percentage of observable aplastics (coarser grained inclusions) and examined the geometric relationships between the aplastics and the encompassing clay matrix (micromass). Grit tempering was used exclusively in the manufacture of each vessel represented. The large and abundant polymineralic grains (rock fragments) are granodioritic in composition and evince local tills as the most likely source for the temper. The mineral composition of the aplastics and the matrix is consistent with raw material resources readily available in central North Dakota. The overall abundance and coarseness of the tempering agent is likely added to mitigate the high shrink-swell capacity inherent in the montmorillonite clays that are prevalent throughout the region. The size, amount, and composition of the temper grains are also common to vessels manufactured for utilitarian (culinary) purposes. It was not possible in this study to distinguish Riggs from Fort Yates ware at the microscopic level.
Active profiling and polishing for efficient control of material removal from large precision surfaces with moderate asphericity
We present the development of an active fabrication technology for controlling material removal on large precision surfaces of moderate departure from a sphere. The underlying philosophy was established as an efficient solution to the challenging problem of fabricating secondary mirrors of up to nominally 2.5 m in diameter for modern 8 m class telescopes and beyond. The facility described comprises a CNC profiler, two contact profilometers, and a full size active polisher. The trial work-piece was a convex surface of 830 mm in diameter on a zero-expansion ceramic (`Cervit') blank – a 1/3 scale hyperbolic mirror for a proposed 2.5 m diameter f/7 secondary mirror for the 8 m Gemini telescope. Using software error-correction from profilometric metrology data, a factor of 2 improvement in generating the convex aspheric profile was achieved. An active loose-abrasive polishing process is also described, in which a full-size tool is configured to deliver variable, edgeless, sub-diameter removal-footprints. Real-time monitoring of process variables is described, and approximately 10% convergence in each polishing pass is reported.