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The 1879 Mathushek square am working on has residual rust inside the agraffe holes, from the really rusty wires.
Been decades since I really had to think about agraffe hole gunk.
Haven't tackled it yet.
Q-tips with 30% vinegar, rubbing alcohol, CLR (lol)?
Tight tubes of fine sandpaper?
All ears and open mind on how to *not waste my time getting them clean enough to string n tune to basic level*.
(PS the budget for THIS tiny step is approx $4.27, so no new agraffes duh)
$4.27 is way too much.

Take 20 inches or so of a heavy piece of core wire (#22 or so) from a bass string clipping and roll the last two inches of it between two files. This will roughen it up a little. Then swedge it about an inch from the end with a hammer on an anvil, (or heavy vise), until it is just a scant bit too wide to pass through the agraffe. This will create a flattened burnisher/chaser. Cut the end just short enough to that there is approx. 3mm of round wire in front of the flattened portion and chuck it up in a 1/4" drill.

The 20 inch length allow you to insert and guide the wire by hand without worrying about getting the drill perfectly lined up with the bore of the agraffe, and the sledging will create a leading "shoulder" to the swedge that leaves a soft exit in the agraffe hole.

You may need two or three of these, as agraffe holes are rarely the same size, even on the same agraffe. You want to be able to pass the swedge through the agraffe with light pressure from the pushing the drill into it. The roughed edges will begin to show a brass tint as it passes through successive agraffe. It can be done without the roughing if one simply wants to burnish, but I found that a very light abrasion does the job faster.
Interesting method Ed. I have routinely used abrasive cord purchased from Schaff. Just run it back and forth a few times with upward pressure and done. Takes about 30 or 40 minutes to do the whole set (and several pieces of cord as they will wear out).

Peter Grey Piano Doctor
If there is oil or grease i would just wash out the string holes with lacquer thinner.

I would take a regular twist bit that is just barely below the diameter of the face of the string hole and grind it so the angle of the bit behind the cutting edge does not slope below the cutting edge. This will make the bit a V-shaped scraper of sorts. Then place a rest block clamped to the drill press table and set the drill stop so the bit just scrapes away enough brass to leave the original string contact surface in the center of the agraffe at about 1mm width.

Then buff the agraffes. And install them after the plate is sprayed gold but before you spray a clear top coat.
Please add your insightful corrections in case my thinking is completely off track, but unless the agraffes are full of gunk AND deformed, the last thing you want to do is to somehow change the agraffes' physical specs.

Identify the gunk's chemical/physical composition and match those to a solvent that doesn't aggressively react with brass. Put agraffes into solution, leave them there for a couple of hours and then clean them with an appropriate tool such as a fitting interdental brushes. Rinse and repeat. Meaning: Keep them as they are and free them of gunk without any physical application of force in those parts where they meet the strings.

If they are already deformed inconsistently from strings of steel being harder than brass, then cleaning might be a short term remedy, but in the long term you will want to change them for new ones anyway.
Stock agraffe string hole specs are wrong. If you champfer the string holes so as to reduce the actual string contact point to no wider than 1mm, they will never buzz. The string will self machine the brass so as to produce the perfect radius for the string bearing angle in that piano.

Piano wire lives and sound best when the full function of the pivot termination principle is developed at the tuning pin end of the string, and the termination point material is softer than the piano wire.

That way the piano wire can self machine the final shape of the termination point and allow a tiny bit of work hardening at the contact point by the initial tuning and spacing process of stringing a piano. The piano string will not cut through the termination point like a cheese cutter.

I have been doing them this way since about 1977 and never any problems. I still tune many of the first pianos I did this on.
I've done it with Q-tips in a Dremel tool with a bit of Brass-o. I remember this changing the shape of the hole a bit, removing some of the deformation from the string. That's a good thing from what I've heard, but it's possible to be too aggressive. (Don't just shove the whole Q-tip in the hole.) The other method I know of is the abrasive cord P W Grey mentioned.
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