Re: Rod Gnomon Hole Placement
y vulnerable to getting rubbed or scratched off. However, weathering, wind, etc. or misguided scrubbing and harsh cleaners can also abrade the fused paint. For that reason, the painted side is usually mounted toward the inside. 2. Are protective glass or acrylic covers on the outside of stained glass windows very common? A Japanese glass artisan told me protective covers are common in Japanese stained glass windows. It is possible to protect stained glass by covering it. Acrylic should NEVER be used. When covering it on the outside - to protect from weathering or vandalism, one must take care that the cover does not touch the stained glass, so the stained glass can expand and contract naturally with the temperature. Also you must leave some airspace around the edges, into the interior space, so you don't get a buildup of vapor as the temperature rises. Today it is possible to send the stained glass out to a professional glass place which will be able to sandwich the glass panel between regular or thermalpane and then create a vacuum - again to be certain there is no water that will collect on the inside. You have no doubt seen panes of failed thermalpane where the condensate is so bad you can't see through the window, and storm windows which hold the moisture in, and the resultant leakage. Improper "protection" will result in accelerated decay, so the panel will need to be restored much sooner that usual. After about 100 years, in well made panels, some restoration is to be expected - mostly the result of gravity. - Cec - Original Message - From: "Andrew James" [EMAIL PROTECTED]To: sundial@rrz.uni-koeln.deSent: Monday, August 04, 2003 9:38 AMSubject: RE: Rod Gnomon Hole Placement John Carmichael wrote (04 August 2003 17:27) What about attaching the gnomon to the building and have it hang outside of the protective cover? I guess that as far as laying out the dial goes that would be equivalent to displacing the gnomon away from the dial by (thickness of cover) * (refractive index of cover - 1). Would anyone like to confirm that or give the correct answer if not? Regards Andrew James -
RE: Rod Gnomon Hole Placement
Patrick Powers wrote (Friday, August 01, 2003 9:10 AM) A propos the matter of the fixing of gnomons to stained glass dials - here's a bit of a conundrum for you. Would you have chosen the fixing holes on the Bucklebury dial? It declines about 13 degrees West yet the holes seem in a very strange place for that orientation... Was the dial perhaps wrongly made in the first place or was there a most interesting gnomon support Perhaps we shall never know. and John Carmichael wrote in reply (01 August 2003 22:01) Looks to me like that dial has big problems too. The 12 o'clock numeral is where it should be for a direct south wall, but the 6 am and 6 pms don't line up with the center. Andrew James was there last week. Andrew, do you remember if this is a south wall? Actually the Bucklebury dial is in a North wall or window :-) . It has been moved from its original site - probably at a (demolished?) house belonging to the Stevens family who lived nearby - to its present position next to the Squire's pew in Bucklebury church. Interestingly the precise cadencing of the shield indicates a Stevens who died earlier in the 17th century so this 1649 dial seems to have been made as a memorial or other remembrance by or for a descendant. It should face somewhat W of S as Patrick says. I didn't notice anything wrong with the delineation on that basis but it has been rather carefully and cleverly offset from the vertical centre line to give a better visual balance to the hours at the E and W edges by moving the vertical gnomon root - XII line to the W (now E, to the right seen from inside). Of course the top right numerals which look like VI and VII are really (I presume) VII and VIII, the final I in each case being hidden by the present leading - hence the odd VI-VI appearance which misleads at first glance. It would seem that the gnomon had a substyle foot as if for a direct South gnomon (which would have to have been folded westwards) as well as a pair of bracing stays, one each side, so that there are three lower fixing holes which is unusual I think - most have only two for a V-shaped foot or stay to the gnomon and omit the third. It is so carefully made and well drawn that I think a mistake is not very likely but that is only my suggestion. However the 1652 dial no. 33 on the SGS web page has a very similar 3-hole fixing - the date is very close and perhaps there is a connection there? The numerals and the cross patty, and the quarter hour marks, dots, and hour lines all appear extremely similar in draughtsmanship and form so would it be unreasonable to suggest the two dials might be by the same hand? I'd like to see a close up of the fly of no. 33! Re Ledbury: The church guide to the windows refers to this very rare 17th century stained glass sundial, and says that for many years it was upside down before the reglazing of this window by the Friends in 1988. I believe that when I saw it (June 2000) it was the right way up though of course that could have been wishful thinking :-) . Regards Andrew James -
RE: Rod Gnomon Hole Placement
John Carmichael wrote (01 August 2003 16:12): Because of the importance of rod gnomon attachment on stained glass sundials and the interest in the subject, I thought I'd make a Delta Cad scale drawing for the website showing a sideview of the hardware used to attach it to a pane of glass, using the bolting method. I should think that fixing a gnomon by just one attachment at its root would be problematic. For one thing it will be awkward to align accurately, but more importantly it will exert great leverage at the fixing point, and if the end is pushed by whatever means it will be far more likely to break the glass. Better to do what most of the old ones did and have a forked stay supporting the bottom end or the middle of the gnomon similarly fixed by nuts and washers into two more holes. That both assures rigidity and placement and ensures that very little strain is on any one of the fixing points, if properly fitted. It may be worth pointing this out to avoid short lived designs! Incidentally the outside of Bucklebury seems clearly to show the washer positions. Have you also considered the difference between drawing the hour lines on the inside and outside of the glass? The diagram shows the centre as outside, but if the paint is inside surely the gnomon root should pass through the dial centre on the inside surface, allowing also for refraction? Regards Andrew James -
RE: Rod Gnomon Hole Placement
I believe the Merchant Adventurers' Hall sundial has a brass or bronze gnomon like a which is fixed to a ring of the same metal surrounding the roundel in the dial on the outside. The gnomon root or dial origin is some way above the roundel and the actual gnomon. I think I can provide a clearer outside picture of that dial (and will send it to John off-list). Almost none of the old dials have the gnomon in situ. Feel free to use my wording, John. Re refraction: if someone painted the glass not in reverse it would need to be inside, though I don't know what is customary. Given that dials can be quite small I think even 3 mm or 1/8 could have a noticeable effect. It would seem simpler, if the Japanese protective cover can be as much as 10 away, to put it outside the whole glass/gnomon assembly, assuming the gnomon can be arranged to project less than that. As long as the cover has parallel faces it would make no difference to the angle of the sun's light. It would reduce the amount of sunlight transmitted at low incident angles - but then perhaps any frame holding the cover will interfere anyway. To have the cover intersecting the gnomon would produce a complication - and why not use it to protect the gnomon as well as the glass? Regards Andrew James -
RE: Rod Gnomon Hole Placement
John Carmichael wrote (04 August 2003 17:27) What about attaching the gnomon to the building and have it hang outside of the protective cover? I guess that as far as laying out the dial goes that would be equivalent to displacing the gnomon away from the dial by (thickness of cover) * (refractive index of cover - 1). Would anyone like to confirm that or give the correct answer if not? Regards Andrew James -
Re: Rod Gnomon Hole Placement
Hello John, A glass artisan in Japan contacted me and told me that it is customary there to put a protective glass or acrylic cover on the outside of stained glass windows to prevent damage. He said that the covers are usually between 2 and 10 inches away from the window and wanted to know it the protective cover would interfere with the gnomon. Well all sorts of red lights went off in my head about complications such as refraction if the gnomon was attached outside the protective cover. I couldn't answer his question and told him to contact Fer! Having looked into the issue of refraction before designing my windowsill sundial, I can report that the refraction in the situation you mentioned should not be a problem. I was worried about the sun's rays being distorted by the window glass before it even hit my sundial. What I found is this - as the ray of light hits the outside surface of the pane of glass in a window it is bent a bit, then it travels through the material at this slightly different angle, then upon exiting past the inside surface of the pane it is bent back to its original angle. This of course assumes a modern pane of transparent material where both the inside and outside surfaces of the pane are parallel (uniform thickness). A hand cast or irregular thickness piece of glass will refract the rays differently, and would be a problem on a stained glass sundial made of that type of glass if the hour lines were on the inside of the window. On my Spectra sundials I do all of the shadow casting activity on the back plane of the glass dialplate to avoid having to deal with the refraction of the light passing through the dialplate. I have placed many sundials over the past year or so, and double or even triple pane glass windows - some with energy/uv coatings - have not confounded the accuracy of any of them. Best Wishes, Jim -
Re: Rod Gnomon Hole Placement
Hi John, Perhaps I was a bit too general in my last posting, because I can see problems if the protective acrylic window was bowed and not perfectly flat. Looking at some of the replacement windows in my garage door, the acrylic ones are not as flat as the glass ones, and I would assume that the rays are shifted as a result. Actually there are optical imperfections in automotive glass too, if you really think about it. I would say that if the outer protective window was of good quality and flat, it probably would not pose a problem...but I don't know about putting the gnomon on the outside of it. That is probably best discouraged as far as the glassers go. Jim -
Re: Rod Gnomon Hole Placement
Yes Andrew, I agree that we should give a little more info on forked or single stay supports. I think I'll even add a drawing and more comments since we don't have ANY good photos of the outside of a stained glass window, with or without stays. This is the photo that the site really lack. Can I use your wording? We've got a somewhat poor photo of Chris Daniel's Merchant Adventure Hall Dial (see tech page on site) from the outside, but it is difficult to see the gnomon. But it looks like it might have a single stay. you wrote a pertinent comment: Have you also considered the difference between drawing the hour lines on the inside and outside of the glass? The diagram shows the centre as outside, but if the paint is inside surely the gnomon root should pass through the dial centre on the inside surface, allowing also for refraction? Most stained glass dials are painted on the outside I believe, so I doubt refraction is a problem, especially if the glass is only 3mm thick. A glass artisan in Japan contacted me and told me that it is customary there to put a protective glass or acrylic cover on the outside of stained glass windows to prevent damage. He said that the covers are usually between 2 and 10 inches away from the window and wanted to know it the protective cover would interfere with the gnomon. Well all sorts of red lights went off in my head about complications such as refraction if the gnomon was attached outside the protective cover. I couldn't answer his question and told him to contact Fer! John L. Carmichael Jr. 925 E. Foothills Dr. Tucson Arizona, USA Tel: 520-696-1709 Email: [EMAIL PROTECTED] Sundial Sculptures Website: http://www.sundialsculptures.com Stained Glass Sundials Website: http://advanceassociates.com/Sundials/Stained_Glass - Original Message - From: Andrew James [EMAIL PROTECTED] To: sundial@rrz.uni-koeln.de Sent: Monday, August 04, 2003 7:16 AM Subject: RE: Rod Gnomon Hole Placement John Carmichael wrote (01 August 2003 16:12): Because of the importance of rod gnomon attachment on stained glass sundials and the interest in the subject, I thought I'd make a Delta Cad scale drawing for the website showing a sideview of the hardware used to attach it to a pane of glass, using the bolting method. I should think that fixing a gnomon by just one attachment at its root would be problematic. For one thing it will be awkward to align accurately, but more importantly it will exert great leverage at the fixing point, and if the end is pushed by whatever means it will be far more likely to break the glass. Better to do what most of the old ones did and have a forked stay supporting the bottom end or the middle of the gnomon similarly fixed by nuts and washers into two more holes. That both assures rigidity and placement and ensures that very little strain is on any one of the fixing points, if properly fitted. It may be worth pointing this out to avoid short lived designs! Incidentally the outside of Bucklebury seems clearly to show the washer positions. Have you also considered the difference between drawing the hour lines on the inside and outside of the glass? The diagram shows the centre as outside, but if the paint is inside surely the gnomon root should pass through the dial centre on the inside surface, allowing also for refraction? Regards Andrew James - -
Re: Rod Gnomon Hole Placement
What about attaching the gnomon to the building and have it hang outside of the protective cover? John L. Carmichael Jr. 925 E. Foothills Dr. Tucson Arizona, USA Tel: 520-696-1709 Email: [EMAIL PROTECTED] Sundial Sculptures Website: http://www.sundialsculptures.com Stained Glass Sundials Website: http://advanceassociates.com/Sundials/Stained_Glass - Original Message - From: J.Tallman [EMAIL PROTECTED] To: sundial@rrz.uni-koeln.de Sent: Monday, August 04, 2003 9:11 AM Subject: Re: Rod Gnomon Hole Placement Hi John, Perhaps I was a bit too general in my last posting, because I can see problems if the protective acrylic window was bowed and not perfectly flat. Looking at some of the replacement windows in my garage door, the acrylic ones are not as flat as the glass ones, and I would assume that the rays are shifted as a result. Actually there are optical imperfections in automotive glass too, if you really think about it. I would say that if the outer protective window was of good quality and flat, it probably would not pose a problem...but I don't know about putting the gnomon on the outside of it. That is probably best discouraged as far as the glassers go. Jim - -
Re: Rod Gnomon Hole Placement
John Your message and especially the attached draw recalls to mind that in my site: http://web.tiscali.it/partena/index.htm I have published a short note (alas in italian!) on the theoretical problem of inserting a cylindrical polar gnomon into a wall. As the lecture of the hour is made with reference to the center of the gnomon shadow, the axis of the cylindrical polar gnomon should coincide with the center of the sundial, that is the point where meet all the hour lines. The intersection of a cylinder (the cylindrical polar gnomon) with a plane (the wall) generates an ellipse. The center of the sundial should coincide with the center of the ellipse. Once you have fixed the gnomon to the wall that coincidence point is hidden. I was interested in calculating the distances of the orizzontal and vertical tangents from that point, in order to use them in the program for calculating the coordinates of the points of the hour lines with reference to a system using those orizzontal and vertical tangents as xy axis. In the same note I pointed out (and this might have some reference to the content of your message) that in the case that the sundial shows also the solar declination curves, it should be taken into account that the material length of the the cylindrical polar gnomon is longer than the theoretical one by: d/(tan lat * cos decl) where d is the radius of the cylindrical gnomon. Ciao Angelo At 08.11 01/08/03 -0700, you wrote: Hello All: Because of the importance of rod gnomon attachment on stained glass sundials and the interest in the subject, I thought I'd make a Delta Cad scale drawing for the website showing a sideview of the hardware used to attach it to a pane of glass, using the bolting method. Before I made the drawing I already suspected I see one very interesting detail. This may come as a surprise to most of you, but the drawing clearly shows that the center of the sundial, where the hour lines converge, IS NOT where you drill the hole in the glass! This is true for all those bent rod gnomons that are not perpendicular to the dial face. And it applies to all sundials with bent rod gnomons, not just stained glass ones. In my drawing, I used 1/8 (3mm) thick glass, 1/4 (6mm) thick rod gnomon, 1/2 padded and flat washers, and 1/4 nuts. You can see that the glass hole is .371 inches (9mm) below the dial's center. This is an important design consideration that the designer must consider in his plans. John John L. Carmichael Jr. 925 E. Foothills Dr. Tucson Arizona, USA Tel: 520-696-1709 Email: [EMAIL PROTECTED] Sundial Sculptures Website: http://www.sundialsculptures.com Stained Glass Sundials Website: http://advanceassociates.com/Sundials/Stained_Glass -- Angelo Brazzi mailto:[EMAIL PROTECTED] http://web.tiscali.it/partena/index.htm
Re: Rod Gnomon Hole Placement
Looks to me like that dial has big problems too. The 12 o'clock numeral is where it should be for a direct south wall, but the 6 am and 6 pms don't line up with the center. Andrew James was there last week. Andrew, do you remember if this is a south wall? John L. Carmichael Jr. 925 E. Foothills Dr. Tucson Arizona, USA Tel: 520-696-1709 Email: [EMAIL PROTECTED] Sundial Sculptures Website: http://www.sundialsculptures.com Stained Glass Sundials Website: http://advanceassociates.com/Sundials/Stained_Glass - Original Message - From: Patrick Powers [EMAIL PROTECTED] To: sundial@rrz.uni-koeln.de Sent: Friday, August 01, 2003 9:10 AM Subject: Rod Gnomon Hole Placement Hi John A propos the matter of the fixing of gnomons to stained glass dials - here's a bit of a conundrum for you. Would you have chosen the fixing holes on the Bucklebury dial? It declines about 13 degrees West yet the holes seem in a very strange place for that orientation... Was the dial perhaps wrongly made in the first place or was there a most interesting gnomon support Perhaps we shall never know. Patrick - -
