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Re: Speculum Metal Mirros ( I Must Be Mad.)

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Posted by david sinden/">david sinden on September 9, 1998 19:40:24 UTC

: I know all the hassles of using a spec. metal mirror but : I want to make one largely out of historical curiosity. : - Same reason I have small pile of tin ore and charcoal : waiting for a dry weekend night when I have nothing better : to do. : I have the metal and I made myself a 4 inch mould but : how do I grind - I was thinking plaster tool or would I be : better off using a second blank as the tool. What grades : of grit am I going to need - it's very soft metal so : I assume it will be easier ( in terms of physical effort) : to grind than glass but does that mean I can start : from a finer grit?

: Ian

From David Sinden.

IAN, Please let me have your E.Mail address and I'll send you as much info as I can on speculum metal. I will also put up a shortened version on the A.T.M Forum for general interest. Very briefly, Speculum metal is not just 'Tin' it is a very precise mixture of tin and copper. The exact correct mixture is (quite suprisingly) very hard and brittle. Many old metal mirror workers had problems with the speculum metal actually cracking and breaking up during working because it is so brittle. Its also difficult to grind and polish. I don't think pure tin would be any good - far too soft. You are taking on a difficult project, but one of very great interest. Not many people have ever seen a speculum metal mirror. I have two here. They are not uncommon in certain museums in England. I worked for the old firm of Grubb Parsons who were a development of the work of Lord Rosse (William Parsons) and his son Sir Charles Parsons (inventor of the steam turbine) The Parsons family took over the work of large speculum metal mirrors after the death of William Herschel. Sir Howard Grubb was an eminent Irish astromomical telescope maker. I was chief optician at Grubb's for many years and responsible for making the A.A.T.155" mirror system. - and others. But I'm a keen amateur astronomer in my spare time. Dont forget your e.mail address. Regards, David. England.

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