Mystery Weft Floats Explained
On several different weaving projects, weft floats have appeared on the back of my fabric after I take it off the loom. I weave on the Louet David loom. I never notice these as I weave. I often make mistakes on the threading, not caught until I am well into my project, but these floating wefts have me baffled. Some span two inches, some half that distance, but they seem random, and fortunately they are rare.
This is happening because the shuttle slips under some of the threads in the bottom shed. For some reason, therefore, some of the threads must be slightly raised when you throw the shuttle.
The David is a unique loom in that when you step on a treadle, you are actually making shafts go down to form the bottom shed. The David does have a shuttle race, so check first to see that all of the threads rest on the shuttle race when you step on the treadle for each shed. Because this is happening to you only occasionally, though, it could be that every once in a while you don't step firmly enough on a treadle and therefore not all of the threads are pulled down to the race.
On jack looms, the bottom shed is formed by threads that are not raised. Looms are usually designed so that the “not raised” threads rest on the shuttle race (or the bottom bar of the beater) and are made even by the race or the beater. On some jack looms, if you increase the tension too much, some of the threads can be slightly raised above the shuttle race, causing floats such as you mention. Countermarch looms have no shuttle race at all. The evenness of the down shed is created by careful adjustments to the tie-up of each shaft to each treadle.