Archeomagnetism of a 19th century pottery kiln near Jordan, Ontario

Abstract

The Jordan pottery kiln near St. Catharines, Ontario, was last fired about 1840–1841. Bricks from the kiln floor have intense NRM's (natural remanent magnetizations) in the range 0.2 × 10−2 to 3.7 × 10−2 emu cm−3 which are directionally stable against alternating field (AF) demagnetization to 1000 Oe (7.96 × 104 A/m). Of 31 specimens tested by the modified Thellier double-heating method, 27 yielded reliable paleofield intensities averaging 1.166 ± 0.092 times the present field intensity. Natural remanent magnetization directions are "streaked" in inclination and shallower than expected. Their average, D = 359.4°, I = 71.3 °(α95 = 2.2°, k = 310, N = 15 samples), differs significantly from both 1845 and 1979 fields in the area. Upon thermal demagnetization to 520 or 540 °C, streaking disappears and remanence vectors systematically steepen. The thermally cleaned mean direction, D = 357.8°, I = 73.7 °(α95 = 1.5°, k = 635, N = 15 samples), is indistinguishable at the 95% confidence level from the 1845 field, but differs significantly from the 1979 field.Part of the "inclination error" and streaking of NRM directions could result from fabric anisotropy or tilting of the bricks, but the greater part probably results from shape anisotropy of the strongly magnetic kiln floor, which deflects the internal field away from the external field and into the plane of the floor during cooling. The internal field is also weaker than the external field as a result of this self-demagnetization. The stronger the NRM, the greater the inclination error and the weaker the apparent paleofield intensity recorded. Thus, high-blocking-temperature fractions of NRM should record a less magnetically refracted (i.e., a steeper) paleofield, as observed. Also observed is approximate correlation between the spatial variation of NRM intensity in the kiln floor and variations in inclination and apparent paleointensity. The shape anisotropy of strongly magnetized horizontal sheets is a probable source of shallow inclinations, scattered directions, and weak apparent paleointensities in many submarine lavas sampled by the Deep Sea Drilling Project.