Hard crust causes?

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Wondering what causes a loaf to have very hard crust - really tough to cut even with a good bread knife!

Does this apply to a particular stage of the fermentation process (PF, BF, proving), or throughout? 

Been reducing the time for my preferment lately and the texture and crust seem improved.

You get less caramelisation and therefore a duller thicker crust. 

If you have been reducing the overall fermenting time then that should improve the final loaf if that was the issue. 

Sounds like you have found the answer. 

As for particular stage it all depends on how much sugars there are available when it comes to baking so like most things in bread baking and getting the it down to the perfect ferment pinpointing one particular stage will be difficult. 

If you extend the preferment timing then one will probably have to shorten then bulk. And then again it depends on how much has been prefermented. If a low percentage then one can increase the bulk if a high percentage then decrease the bulk and so on. 

In other words the more you borrow from one stage you will need to shorten the other and vice versa. A higher preferment then shorten the bulk. The lower the preferment then the longer the bulk. The longer the bulk the shorter the final proof and so on. With each stage added the other stages have to be toggled. 

Whatever you do and whichever stage you toggle as long as it is not spent by the time it is baked you'll get a better loaf. If perfectly fermented you should get a golden thin crust.