Soft, light bottom crust and denser bottom

Toast

Hi everyone, all my sourdough loaves have light-coloured bottom crusts and a denser crumb at the bottom. 

I’m wondering if this is because I’m not baking on a surface that is hot enough. I‘m baking directly on a baking tray left in the oven as it preheats for about half an hour.

Here are some photos. The one with the less open crumb is one of my first loaves, and the ones with the more open crumb are my recent bakes. But you can see how they all show the same issue of a light bottom crust and denser bottom. Would appreciate any advice!

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Sorry, but do ignore the strange lopsided shape of the last one there - there was a sticking issue as I was transferring it into the oven. Though, I also do wonder if the loaf is also showing signs of under/over-fermentation? For added info, I do a 1h autolyse, 3h bulk fermentation, and 2.5h final proofing in a very humid 75F environment (tropical country). Thank you for all your help!

I would just move the loaf more toward the bottom of the oven to get more heat on the bottom of the loaf. Other that that you've got a nice looking loaf there.

I agree with Phaz, bake it on a lower rack in your oven.  The pale bottom crust is a sign that the bottom isn’t hot enough.