What is transfer color and how should it be used when fitting parts?
Transfer color is a substance like Prussian blue, candle soot, or bone black mixed with a little oil, applied in a very thin coat to a metal part before inserting it into the wood inlet. It shows where the part is contacting the wood and preventing it from seating fully, so that spot can be trimmed. A common mistake beginners make is applying too much, which makes it hard to tell exactly where the part is rubbing.