Problems with bridges occur mainly when strings are loose and need to be tuned up. The stretching of the strings being tuned pulls the top of the bridge toward the fingerboard. If left in this position, the downward pressure of the strings can warp the bridge, and repeated tuning using the pegs without re-aligning the bridge may cause the bridge to break or fall. We recommend you get a personal demonstration by a violin maker on how to straighten the bridge.
Not everyone uses the same method of bridge straightening. At Robert Cauer Violins, we move the bridge by holding it with our thumbs, as shown here. We lay the violin on the lap with the scroll to the left and the tailpiece to the right. Next, we reach over the bridge, keeping the thumbs on top between the middle strings pointing toward each other.
The index and middle finger reach over from the other side of the strings under the bridge. On the left side as we press with the middle finger against the fingerboard, we can create counter pressure with the thumb against the bridge and move it backwards. So that it does not accidentally go too far, we hold the right fingers in mirror position holding the middle finger against the tailpiece. The fingers should not press against the highest string or it could break. Since only hand muscles are used, the pressure is cancelled between fingerboard (tailpiece) and bridge. The instrument should not move.
If the bridge does not move easily enough, we lower the strings one by one, lubricate the notch in the bridge with the graphite of a soft pencil, and re-tune. Keeping the tuning one note below normal also helps when moving the bridge.
Inspect the strings to see if they are in good shape where they contact the bridge. If the winding has separated, the string can act as a rasp and dig into the bridge. If this is the case, the string needs to be replaced. (For more information, see Replacing Strings )
The fingerboard height and tilt are important preconditions for the correct fitting of a bridge. If the bridge is too low or angled down too much towards the treble string, there will not be enough margin for the bow to play without hitting either the next string or the C-bout edge of the instrument.
If one of the two middle strings cannot be played without hitting one of the neighboring strings, the bridge curve is wrong or the string has cut into the bridge and needs to be raised. By holding the violin in a way that the A string is visually hidden behind the G string, you can see how much higher the D string is. This is the margin the D string has. Both the A and the D string should have a similar margin.
For the photographs, a cello was selected to make the demonstration easier. Here, the D string is hidden behind the C.
If all of the strings suddenly seem higher, be sure and check immediately to see if the neck has become loose. Look for a small unvarnished line along the neck heel where it is inserted into the upper block.
See if the purfling groove becomes wider under the back button.
In either case, loosen the strings immediately, since there is danger that a sudden implosion can occur in which the fingerboard is slammed onto the top with such force that the impact can crack the sound post, the bass bar, or both. Take the instrument to a violinmaker for repair.