Hi John and all, Be careful here. The chemistry is specific to the rock. Edley describes experiments with "vitreous ceramics", specifically alumina. Corning Ware is made from this tough stuff. I believe your rock is sandstone, a sedimentary rock. Although this is mostly silica sand grains which are impervious to acids, the sand grains are cemented together in a matrix. The cement may be silica and the rock a quartzite, on more likely it is a carbonate, limestone, dolomite or iron carbonate. These carbonates are decomposed by acids. If this happens, the cement between the grains is destroyed and you get rapid erosion. Limestone, marble, sandstone, slate, granite and the serpentine "Picture Rock" from Midway British Columbia are all different chemically. These are listed in order of reactivity to acid attack, first on the list are attacked by acids, the latter are not.
By the way the information that Patrick Powers provided on "Picture Rock" was precise but not accurate. Right name but wrong quarry. The "Picture Rock" from Midway BC is an igneous rock, serpentine with quartz veins. Gold prospectors were looking for the mother lode in these epithermal chalcedonic quartz veins. I believe your rock is sedimentary, a sandstone with iron (hematite) staining and veins from an Arizona quarry. Roger Bailey Walking Shadow Designs N 51 W 115 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of John Carmichael Sent: October 30, 2002 7:29 AM To: [email protected] Subject: Re: Eroded rock Sandstone dust has a very salty taste. Does that mean I should dip my stone blanks in acid before I engrave them to protect them from further erosion? A scary thought! When stone masons lay flagstone flooring, they often clean the unwanted calcium carbonate deposits off the stone using dilute sulfuric acid and a scrub brush. But I have always been afraid to use H2SO4 on my stones, fearing that it might damage the stone internally or cause the sedimentary stone layers to separate. I worry that this possible damage might take years to show itself. (But I observed no immediate damage to the stone when I experimented with H2SO4, and it did do a very good job of cleaning the stone.) So you also have to be a chemist to make sundials, huh? John John L. Carmichael Jr. Sundial Sculptures 925 E. Foothills Dr. Tucson Arizona 85718 USA Tel: 520-696-1709 Email: [EMAIL PROTECTED] Website: <http://www.sundialsculptures.com> ----- Original Message ----- From: "Edley McKnight" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Tuesday, October 29, 2002 5:34 PM Subject: Re: Eroded rock > Hi Dialists, > > Awhile back, researching machining on vitreous ceramics, I came upon the statement > that most rock/ceramic surfaces have a much lowered surface strength when the > surface was alkaline and a much greater strength when acidic. (It was in a NASA > report, but I don't remember the number ) Experiments showed that indeed drilling > speed was up to 7 times faster on alumina ceramics while in a solution of a mild > base such as borax than on a neutral surface. A mild acidic solution such as boric > acid doubled the time of drilling. Shear testing and microcrack examination showed > that the acid surface actually was both stronger and healed many microcracks. Even > mild acetic acid made a great degree of difference. > Shaping with sand-blasting obeyed a simular effect. i.e. The surface chemistry of > the etched lines and their surrounds may be acidic enough to hold up to erosion > longer. > > Just a thought. > > Edley McKnight > > - > - -
