I had occasion to retrofit a nice contemporary Gretch with a new bone nut recently, and thought I'd discuss my thoughts on this important component of playability and tone. The owner and I scratched our heads for a minute or two, trying to decide if this was factory-original equipment or not. It's a synthetic compound and there wasn't a whole lot of care put into shaping it. It functions. He didn't think it looked quite right.
It's flat on top and doesn't echo the radius of the fretboard. It's deeply slotted in parallel lines. (For reference, those are *big* strings. 12-52's.
The nut isn't solid, and it's held in place by some adhesive. I prefer to use less glue. One shouldn't need to hammer repeatedly to crack it free. Just a little rap with a block of wood and the amount of pressure you'd use if you were knocking on a door.
That's less than ideal... The ragged take-off point give the string a mushy and inconclusive place to start. They're buried pretty deep, too.
Clean off the old glue!
The slot varies in width from .227" on the treble side to .239"on the bass. It's good to measure these things before taking a file or sanding block to the bone blank. -Make the bone blank an even thickness and by the time it fits nicely in the tighter side of the slot it would be flopping around on the other side. It's not hard to match a little taper like this and it's less stressful than trying to score and cut the nicely lacquered headstock overlay.
Filing to width involves scribing a line to work to, then repeated trial fitting until it feels right.
The pencil has been planed down to half it's usual thickness. It rides the surface of the frets and acts as a guide for slotting depth and preliminary shaping.
The new nut has a more graceful profile that follows the shape of the board, slightly higher on the bass side.
The bass strings only need to be seated about half their diameter. Excess material surrounding them can hang up and cause tuning issues. On bone or (legally obtained) ivory that material can be brittle and crack off. All edges should be nicely radiused with no sharp corners! The profile should be smoothly rounded and provide a good bearing surface, not too long, not too short.
Slanting the string slots a little towards their respective tuning posts helps tuning and prevents premature wear. Two tiny drops of CA glue hold it in place, ready for play!