Hello Frank & others I believe that the carbon coating which covered the old sandstone Dalton dial actually provided some acid rain protection. Since the rain must be in direct contact with the stone to chemically react with it, the carbon coating acted as a barrier slowing down the reaction rate. (Just like the old patina on brass and copper protects against further corrosion). If the stone dial was cleaned and left in the raw unsealed state, it will degrade even faster than before.
I am assuming that "water repellent" means an epoxy or polyurethane sealer (It could also have been a wax or oil, but I doubt the restorers would have used those.) Of course, these are much better reaction barriers than a carbon coating. A good sealing job will provide 100% protection against acid rain. These sealers will not only prevent water from wetting the surface, but since they penetrate deeply into the sandstone pores, they protect the interior of the stone as well. However, sunlight will eventually degrade the sealer on the stone's surface and it will flake off after several years. The dial will then have to be lightly sanded and a new coat applied. (However, the original sealer located deep in the stone's pores remains good and will always offer additional protection whether the sealer is reapplied or not, since it is unaffected by sunlight). It is my opinion that new and old sandstone sundials should be sealed for acid rain protection. But a note of caution! Stone must be as dry as possible before a sealer is applied!!! Even though a stone may look dry on the surface, water is always present inside sandstone unless the stone has been baked in an oven. A sealer can trap water inside the stone and cause it to fracture from freeze/thaw temperatures. Since it is impossible to dry the stone on a wall in an oven, it would be best to apply the sealer only after a period of hot dry sunny weather when the stone is at its driest or use a heat lamp. Hope this helps. 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: "Frank Evans" <[EMAIL PROTECTED]> To: "John Carmichael" <[EMAIL PROTECTED]> Cc: <[email protected]> Sent: Sunday, November 25, 2001 9:44 AM Subject: Re: ancient dial > John Carmichael wrote; > > >Hello Frank > > > >I'd like to help you with your question, but first, do you know the type of > >stone in which the Dalton dial is carved? Also, by water repellent do you > >mean a sealer such as polyurethane? > > > > > Reply to John: I believe the dial is of sandstone. I don't know what > water repellent was used as I have only a copy of the conserver's > proposals (the same man for both the Escomb and Dalton dials). He wrote > only: After conservation a water repellent must also be applied and re- > application of this water repellent coating is necessary at 5-10 year > intervals to maintain the protection of the underlying stone in an > exposed environment. > > However, no note of a water repellent appears in his account of the work > finally done. I hasten to say that although I was involved in the > initial proposal to protect the dial in some way I have nothing (other > than interest) to do with this work. That is in the care of the parish. > But acid rain has become a destroying demon in this part of the world. > Thanks for your message. > Best wishes, Frank. > -- > Frank Evans
