The bridge may also be bowed lightly to produce an air sound. This may be executed with a regular horizontal bow stroke with the hair of the bow split over either side of the bridge, or with a vertical bow stroke, with the bow perpendicular or diagonal to the bridge, to decrease the likelihood of activating the strings.
There are numerous examples of bowing on the bridge in composition for modern string instruments over the past few decades, including in Krzysztof Penderecki’s Threnody to the Victims of Hiroshima (Penderecki, 1961, p. 19), and Helmut Lachenmann’s work for solo violoncello, Pression (Lachenmann, 1969/2010, p. 2).
Bowing the bridge on the baroque violin:
Bowing the bridge of the baroque violin produces a similar timbre to that of the modern violin. However, baroque violin bridges usually have a flatter arc than modern violin bridges, which can make maintaining even contact between the bow and bridge simpler on the baroque violin than on the modern violin. Furthermore, gut strings require more careful bow contact in order to resonate, making any accidental contact between the bow and strings while bowing the bridge less likely to result in unintentional activation, or squeaking of the strings. One complication that may arise when bowing the bridge on the baroque violin is that the bow hair on an unmounted frog is not as voluminous as on a modern bow, and can more easily bunch together, reducing the surface of hair in contact with the bridge, and consequently reducing the potential dynamic level of this technique on the baroque violin.
The following is an example of horizontal bowing on the bridge on the baroque violin, followed by vertical bowing on the bridge, producing an air sound.
Video example 12.1 – bowing on baroque violin bridge, horizontal followed by vertical
In some instances, the term molto sul ponticello or m.s.p. may be used to indicate playing on the bridge. However this term may be ambiguous, implying an extreme sul ponticello bow position where the bow is still on the usual playing side of the bridge, thereby activating the strings, albeit with a great deal of distortion. To avoid ambiguity, if an unpitched, bowed wood sound is desired, an “x” note head on a single pitch with “on bridge” added above the stave may be employed, as in Figure 12.1 below.
Figure 12.1 – example of a possible notation for bowing on the bridge.
 For a comparison between bow hair in unmounted historic frogs and modern frogs, see Figure 9.1 in the Catalogue entry entitled Tremolo, vertical spazzolare, arco circolare.