Like Hide Glue, thick or “burnt” shellac is another “sorta kinda” natural substance, the use of which is traditional in this industry and has been sanctified by time.
While it is undeniably interesting and fun stuff, it’s also just as sticky and messy as glue, so we don’t want to get too playful with it!
As the name suggests it’s simply a very thick formulation of regular, everyday shellac finish. In player piano actions, its primary use is as a sealant. It sticks to absolutely everything, which is good if you put it only where you are supposed to! As the carrier solvent evaporates the shellac slowly cures to a hard shell.
So it can be “painted” inside of block channels or also around the junction of metal bits into wooden blocks, etc. Once cured it is airtight, and won’t come off, unless you want it to. And here again, it is wonderfully “undoable” like hide glue.
Burnt shellac is also good to reinforce a fastened joint, which can’t be glued but still need strength. An example of this would be for the arm flanges of a motor. The flanges attach to the bellows with small screws, but as they are under a constant load, they need a little extra strength as insurance. However it’s not a good idea to glue them (as they may need to be removed for repair purposes), so shellac is a good compromise.
This wonder substance can be made in two ways: additive or subtractive. Note that the finished product is not exactly the same in both cases, as explained in further detail here.
The additive way is slower, but safer. Buy yourself a bag of shellac flakes (from a woodworking supply store), and gradually add a minimal amount of solvent (e.g. mineral spirits), just enough to dissolve the flakes. If you can get it right you will have a thick goopy mixture.
The subtractive way is the traditional way, as the other name suggests. Premixed shellac is bought from the hardware store, then “burned off” by lighting the liquid on fire, and letting it burn for such time as that it thickens to a goopy mixture. It will cool to a thicker consistency than that which it burns, so don’t overdo it with the burn off!
For this method it is highly recommended that you do this out of doors away from combustible surroundings.
Disclaimer: I take no responsibility for any cataclysmic events that occur after the reading of this post!