Well what a pleasure an easy crossword for a change. I've not been feeling well (a bad and lingering cold), so it was good to sit down to a crossword I could solve in good time for me.
There are three I can't fully parse: 17ac, where does the back and forth come in? 4ac and inevitably 7/8d. I've looked at the hints above, but am still in the dark. I got the latter straight away, but simply cannot see the chemistry or the Roman numerals in any meaningful way.
I had to check out clay ovens to get 13d, which I took to be a simple anagram with a missing letter.
My FOI was 6ac which I thought was very clever; LOI 4ac and my CODs are 22ac, 10ac and 6ac. I suspect I would add in 7/8d if I could find the cunning parse.