I recently received a horn for restoration and was impressed with the look of the previous overhaul: cleanly beveled cork pads on the upper joint, and tan kid on the lower. The only problem was that it didn't pull anything close to a vacuum: Those beautiful cork pads had been laboriously installed on a 1920's Albert system clarinet, whose tone-holes were nowhere near flat or sharp enough to make a clean impression...and, hence, a good seal!
This got me thinking about the pros and cons of cork pads; many people request them in an overhaul, often without any clear reason for doing so other than that they heard it was the 'thing to do'. On a modern horn with crisp, flat tone-holes cork is a joy, giving a great seal and improving the clarity and projection of dull notes. On anything older than a couple decades, I will generally try to discourage them from full cork on the upper joint, and here's why:
A tone-hole is cut flat into a curved, wooden surface. Wood changes over time, flexing, swelling and shrinking with variations in temperature and humidity. A new tone-hole will be flat and sharp, if you're lucky - an old one is often anything but! Aside from changes in the wood itself, there is also the quality of the initial machining and subsequent overhauls to consider: Hasty/careless work will sometimes leave small nicks/dings on the edge, and sometimes you'll see evidence that someone has attempted to 'reface' a tone-hole, doing more harm than good.
In these circumstances, a cork pad is more of a liability than an asset! Its firm surface, well suited for taking a clean impression from a perfect tone-hole, won't contour to an imperfect one and the result is a poor seal. This can be hard to diagnose subsequently, as the ring that cork takes will fool a paper feeler with its bite, leaving you scratching your head as to where that leak is coming from!
On the 50+ year old instruments that I commonly service, I'll use cork on the register key (since its tube doesn't change with age) and maybe A/Ab if the throat is stuffy and the tone-holes look good: often they'll need a little 'touching up' to pass muster. Other than that, I stick with leather, which is long lasting, contours well to the aging tone-hole, and has a feel more similar to the fingertip for even response. I'd be interested to hear about other people's experience with cork on older horns....
The Licorice Shtick Blog is the creation of the Vintage Clarinet Doctor, a Winston Salem, NC based woodwind instrument repair shop specializing in vintage and antique clarinets, saxophones, and the occasional flute.