Paul, I have an old house that had some wood finish like that. I'm not sure what caused it. I kinda thought it was just old. You might try soaking a rag in denatured alcohol and put it on an out-of-the-way spot to see if it softens up. If it does it's shellac. You should be able to get it off with the alcohol and re-shellac. But, like Steve said that one area will look a lot different than the area you don't do. I re-did all mine and it looks great. . . needless to say, it was a huge job. Don't forget, be careful with alcohol fumes inside and take the rags out when your finished.