Thanks Tony and Luke for the excellent advice on this topic. This is a thread well worth continuing. Thanks Guido for the good question.
There is a technique called "intaglio" for scratching lines in fresh mortar to expose a dark layer underneath. I have seen beautiful designs, including sundials, decorating houses in Switzerland using this technique. Can anyone provide further information? Woody Sullivan has provided a solution for the problem of the two planes created by using raised hour and declination lines. See <http://www.astro.wahsington.department/tour/sundial>. This remarkable sundial is a large structure designed to be read from a distance. In general use, the shadow of the gnomon style between the hour lines on the plane of the wall tells the time. Accurate readings are possible when the shadow falls on the hour or declination lines. Here the spherical nodus at the end of the gnomon is used to project the correct shadow. Not only is the sphere the right size for the proper umbra projection but careful inspection will show that the end of the gnomon style is bent, projecting out just enough for the shadow of the sphere to be in the plane of the hour lines and not the plane of the wall. A clever solution. Thanks Woody. Roger Bailey Walking Shadow Designs N 51 W 115 >Tony Moss wrote: >Arising from Luke's suggestion a word of warning to the unwary who might >omit to fill the space between the hourlines as he suggests. Don't >forget that if you use hourlines which are raised above the dial surface >by any significant amount then the origin/root of the shadow-casting edge >must lie in the same plane as the *top* of the hourlines and NOT the >plane of the dial plate on which they are set. > >In effect there will be two dials one above the other with the shadow >'climbing' from the lower one up the side of an hourline before arriving >'late' top centre of the upper dial hourline. Such arrangements are >always a compromise. The same applies to deep cut broad hourlines but in >reverse. > >
