My house was built in 1970 or 1971 (I've seen both years, can't remember which one is correct), and the bathtub and the bathtub walls are original to the house. The caulking has been coming off, so I started removing it to replace it. Below about four layers of still-pliable caulking, I discovered a very hard layer of...something. It looks like it's either extremely dried-out caulk or possibly a type of plaster, or maybe grout. I've been removing it since it's moldy in some places and so I can make a clean, fresh reapplication of caulking. (If I shouldn't be removing it, let me know.)
However, it's been very difficult to remove this hard stuff. I've had to use a screwdriver as a chisel and hammer the stuff out. It breaks apart in pieces when I do this, so not terrible, but it is taking a while. But worse than that, my tub is porcelain-covered cast iron, and I noticed that layers of the porcelain are coming off when I hammer this hard stuff out, so I've stopped. None of this porcelain damage has exposed the cast iron, so I think I'm good so far. The walls appear to be some kind of resin and are also sustaining very slight damage from the hammering.
How can I remove this hard stuff, this plaster or dried-out caulk, in a way that doesn't damage my tub or wall and preferably is quicker than what I have been doing?
I should add, I tried hammering with a chisel with a wider blade (a paint spatula), but that spreads the force out over too large of an area it would seem, it doesn't remove any of the hard stuff.
Photos. First, close-ups of some of the chunks of the hard stuff that I have pulled out. Don't let my poor white-balance fool you, this stuff's color is white, not yellow. That being said, the darker portion of the longest piece is actually gray.
Now, a spot where the original caulking still remains, transitioning into a spot where I've removed the stuff that I can easily pull off, with the hard stuff remaining.
The hard stuff remaining in the wall-to-wall gap, and below that point.
What the gap looks like with everything removed (some small bit of soft caulk and hard stuff remain).
The whole bathtub for perspective.