Roger: That's an interesting theory explaining why a stone's engraving weather less than the surrounding stone and it might be true, but I doubt it.
You are correct about dust being created in the carving process and about it getting into the surface pores, but it has been my experience that this dust is very easily washed out of the pores with just a little water. A light rain would do that. So I don't think dust in the pores is significant. But you are also correct that erosion is principally caused by freeze/thaw periods. Here's my theory: Stones are big "Heat Sinks". They store heat well for a long time. When the weather begins to freeze, the inside of the stone stays warm and the outside of the stone freezes first. Therefore the face of the stone is expanding and contacting more than the warmer inside. Since the engraving are closer to the interior they are subjected to less stress. If my theory is correct, then this effect would show up on all types of stones. 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: "Roger Bailey" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Wednesday, October 30, 2002 10:25 AM Subject: RE: Wind erosion of incised lettering. > Bonjour Alain et tous amis, > > Pardon, mais je ne parle pas Français. > > I think your information is the best explaination we have heard so far on > why the rock base sometimes erodes faster than the carved letters on old > sundials and tombstones and this effect depends on the tool and technique > used to carve the stone. Let me explain my theory and why I agree with your > friend the stone carver. > > First, I believe that the rocks that we are dealing with are sedimentary: > sandstone and limestone. These rocks are quite porous, with porosities of up > to 30%. They are also permeable. Fluids like water, oil and gas can flow > through them. When I look out my window at the towering peaks of the Rocky > Mountains, it is hard to believe that this rock is full of holes and fluids > flow through them but it is true. These is the same rock formations that are > the oil reservoirs that are the economic base for the Province of Alberta. > > Second, the greatest cause of erosion is the freeze/thaw cycle. When water > freezes, it expands. This cracks the rock structure just the same as it > bursts water pipes. The rock surface is weakened by this repeated freezing > and thawing and easily sloughs off in the wind, rain, and running water. > Local conditions apply. It does not freeze in the tropics. River and glacier > erosion is rapid where it occurs. None of these apply to Tucson! > > Third, when the rock is worked with impact tools, the fine dust created by > the impact can be pushed into the voids, lowering the porosity and > permeability in the affected area. With less water able to get into this > area, less damage is caused by the freeze thaw cycle so these areas remain > hard and durable, suffering less erosion. When the rock is worked with > cutting tools, the dust is cut away and less gets pushed into the voids so > less protection from erosion is provided. > > This theory predicts that sandstone and limestone would be most affected, > slate and marble less and granite not effected at all. Does this match the > observations? > > Roger Bailey > Walking Shadow Designs > N 51 W 115 > > Where we are in the freeze part of the cycle, -18 C this morning! > Conditions are quite pleasant here in in August. > > > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of Alain MORY > Sent: October 21, 2002 1:42 PM > To: INTERNET:[email protected]; [email protected] > Subject: Re: Wind erosion of incised lettering. > > > "Bonjour à tous," > Hi fellow shadows watchers ! > > > "Je dois dire que je suis très gêné pour répondre en anglais, car mes mots > sont trop limités pour ce sujet précis." > I must excuse me for this quite bilingual answer ! > the topic is very precise, but I guess that my words will not be quite exact > to say what I mean. > > I'm working with a sandstone hewer (or carver) ("un tailleur de pierre"), > and he gave me two reasons for this unusual erosion : > > 1 The force given to the cutting tool enters in the stone. Small pieces of > stone are gone away (the work of the hewer itself !) > and other part of the force is making little "amorces de rupture" in the > stone. this will reveal only years later, with erosion of > wind, water, ice.... > > 2 Between the lines, the hewer decided to use special tool, ("gradines, > marteline, boucharde") made of multiple teeths, to add > an effect on the stone, or to . This effect will also facilitate the work of > erosion, with centuries. > > Friendly yours > > > > > 21/10/02 18:51:43, Patrick Powers <[EMAIL PROTECTED]> a écrit: > > >For those who might be prepared for a 200kB+ download there is an example > >of the sort of dial erosion I was referring to on my web site. Set your > >Browser to > > > >http://ourworld.compuserve.com/homepages/Patrick_Powers/dialer~1.jpg > > > >This dial is dated 1691 and has incised lettering but raised hour lines! > >My theory has been that this is a consequence of the lines having been > >painted and that this prevented, to some extent at least, the stone being > >eroded there. I don't see any other selective erosion on this dial that > >might point to someother effect but I am not an expert in such matters. > > > >Not sure if this is is the same sort of thing that you saw Tony or whether > >anyone can offer another explanation... > > > >Comments please!! > > > >Patrick > > > >------------------------------------------------------------------------- > >Forwarding addresses: > >E-Mail: [EMAIL PROTECTED] > >Web: http://www.dunelm.org.uk/homepages?patrick_powers > >Lat: N 51d. 49m. 09s: Long: W 00d. 21m. 53s > > > >- > > > > > > > > - > > > - > -
