I bought this vintage calculator this week in a sealed bag at a thrift store. I knew the risk, but it was super cheap and a great looking device.
When we opened the battery compartment, we had quite the surprise.
This Calculator has the original Craig branded Nickel-Cadmium rechargeable batteries from the 1970s. The buildup was the consistency of the calcium you would find in a shower or something, it was very difficult to remove and clean.
Sources:
https://www.calculator.org/calculators/Craig_4506.html