In general, marble is not favoured for exterior stone work in UK - pollution and all that, not to mention algae etc. Surprisingly, black/dark slate with a matt finsih gives a very good surface for shadows. David Brown Somerton, Somerset, UK
-----Original Message----- From: Roger Bailey <rtbai...@telus.net> To: sundial@uni-koeln.de Sent: Wed, 18 Mar 2009 3:43 am Subject: Translucence You may think, as?I did, that white marble is an ideal base material?for a sundial. The rock is relatively easy to engrave; the white surface should provide good contrast for the shadow. ? This is not necessarily so. I found from tests with a prototype that the translucence of marble, that wonderful milky quality so favoured by sculptors, does not show high contrast shadows. The light from the bright areas seems to diffuse to the shaded zone and the shadow contrast can be greatly reduced. The effect is similar to the shadow formed on a hazy day when thin clouds diffuse the sunlight. ? My prototype was a marble tile, 30 cm square, 1 cm thick. I could see the outline of my hand through the rock. One side was polished and glossy, the other a rough matte finish. The glossy surface of the translucent material did not show high contrast shadows even in bright sunlight. The matte finish was significantly better but still the contrast was lower that the shadow of a white piece of paper. The prototype was a sunset sundial and the shadows approaching sunset tend to be diffused. The translucent base made them invisible. ? This experiment reminded me of a previous experience with stainless steel. Again the shadow contrast on the shiny surface was poor. Shiny and translucent? Both are bad for shadow contrast. ? Regards, ? Roger Bailey www.walkingshadow.info --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
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