Page 327 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

306 Maritime Archaeology: A Technical Handbook, Second Edition
to a predrawn axis. It is, therefore, easier to trace around the pot and then set the axis. Also, one needs to be patient and possess the skill of a contor- tionist, because it is essential that the pot must not move. This requires one hand to hold the pot at all times, while the other hand operates the profiler, which has to go all the way round the hand holding the pot. So it is impor- tant to think where you want to start.
Another profile device is an array of thin strips of wood, plastic, or metal in a holder. The strips are pushed against the side of the pot thus duplicat- ing the profile. The profile machine is then laid down on the drawing surface and the profile traced out (Figure 11.7b). It is much easier to operate than the previous system, but it only makes a profile of one edge of the object and often only part thereof, so that additional measurements need to be made to get the profile in the correct orientation with the axis of the pot. Both machines described above are cheap and easy to make with minimal tools and a bit of skill.
Slightly more complicated to make, but really useful, is the profiling stand (Figure 11.8). This instrument consists of a flat baseboard with a ver- tical post with a ruler attached. Sliding up and down the post is a block that can be clamped in position. The block has a horizontal slot in which another ruler is set and can slide in and out. The horizontal ruler has a pointed end and this is set against the object to be profiled. To profile an object, it is placed on the baseboard with the profile in line with the vertical post. The horizontal ruler is set against the profile at regular vertical increments and the horizontal and vertical measurements taken. The great advantage of this system is that the measurements can be transferred directly to a drawing using a drafting machine.
It is often necessary to determine the radius of curvature of objects, par- ticularly pottery wall shards, so that a reconstruction can be effected in the drawing. Simple trigonometry and a hand-held calculator can simplify this and give accurate results. Essentially, it is necessary to know the amount that the arc is displaced between two fixed points A and B (Figure 11.9b). Thus, if two fixed points are distance 2 D apart and the maximum distance from the straight line between these two points to the wall of the object (the chord–arc distance) is d, then the radius of curvature R is given by:
Care has to be taken to ensure that the chord–arc distance d is to be a point that is in the plane of symmetry of the chord. For example, the orientation of the shard to the axis of symmetry is correct when taking the measurements and that the chord lies in a plane at right angles to the
R=
(D2 +d2) 2d