fireplace oven

I remember a similar oven used in Wales. Every part of it was made of iron to withstand the heat. I doubt anything else would survive for any length of time. I believe the "plug" you spoke of was a piece of iron basically the same size as the oven side and it was placed between the oven and the fire.

I don't know if this will be of use -- in earlier times, ovens were often made of clay or stone. I suppose it could mean that later "plugs" were made of slate.

Fire oven

My grandmother had a fire oven very similar to the one you describe.

Her house was in Yorkshire, too. She used to cook using this oven in the 1920s and 1930s.

Every part of the oven was iron. She used to "black" it every week right up until she moved out of the house in the 1960s.

You could shield the oven from the fire with an extra plate of iron. This was the only way you could adjust the heat and bake things at different termperatures. After all you always needed a good fire in the parlour to heat the house - no central heating in those days, and no heating in the bedrooms, - so all the heat for the house came from the one fire.

I am told that bread bakes well in these ovens but can't speak from experience.

I've only seen iron used for these. Sometimes it's a door (on hinges), and sometimes it's a rectangular plug or plate that is removed and inserted with the help of a small handle (or knob).