Originally Posted by
Stanley Covington
There are times when a curved line is necessary. It is a very advanced technique that I have never had occasion to use (not even sure I could do it successfully without a lot of practice), but I have seen it done by carpenters replacing trim and long, large beams in temple/shrine repairs/reconstruction.
The trick is to twirl the inkline between thumb and forefinger when lifting it, and pull it just the right amount to the side. When released, the combination of twist and offset causes the inkline to leave a perfectly curved line on the wood.
I am aware of three uses for a curved inkline.
First, if the eye can see a long percentage of a horizontal beam's or trim board's or hafu's exposed length, for instance on the building's exterior, and the member is perfectly straight, the human eye will see it as sagging or "dead." By applying an upward curvature, it appears straight and "alive."
The second application is to create a slight curve in roof rafters. This is an appearance factor, once again.
The third application is more structural. Long, unsupported, horizontal beams always deflect downward due deadload, liveload, and creep. Some of the sag occurs immediately, some after the building is occupied, and some takes decades to gradually develop. Often this sag can be hidden behind ceilings so it is not apparent, but not always, especially at\above doors and windows. Other times, a sagging beam, especially over doors and windows, will sag and cause joinery to bind. And it just looks "tired" and worn-out.
I have spoken with top-class carpenters in Japan as they were marveling about the skills of the carpenters who's work they have inspected/repaired in some of the famous wooden structures in Japan, mostly Nara and Kyoto. The gradual increase in deflection over time can be seen in the repairs made over the centuries to fragile plasterwork above and below these beams, indicating that the beams did indeed sag, exactly as the original carpenters expected, but still retained a slight upwards curvature.
Those old boys were looking down the road centuries as they snapped their curved inklines.
Stan