Looking at cell bars

These two black and white photos we can take a good look at the bars that make up the walk way and the cells.
Since the 1930's the bars of prison cells have been tool proof. Before this the bars were made of so called (soft steel). That a number of inmates over the years worked on, and then escaped from.
Then in places like Alcatraz in 1934 they used steel bars that had a core hard center  that no tools could cut through.