Oven Built, working, but loses heat

[cgfw201]

Hi there,
Spent the early part of this month building a clay oven. I broadly followed the guide here (https://clayoven.wordpress.com/).

Basically built a brick base.
Filled the base with chalk (had a load spare)
Levelled that with sand
Layer of glass bottles on top of sand
Sand on top of that
Layer of bricks on top of that as base of oven floor.

The dome is made of a 5-6cm layer of 2:1 sand:clay mix
Then a 5cm layer of clay slip mixed with straw for insulation
And an outer 5cm layer of 2:1 sand:clay mix

If i build a fire with kindling and slowly adding to it, it reaches 400+C in around 3 hours. I can cook a couple of pizzas. in 90 secs or so before the temp starts to drop again. Yesterday I tried to do 10 but had to take a break after 4 and rebuild the fire and leave it for 30 mins to heat up again. It went from 400 to 200 in the space of about 5 minutes.

I'm sure the answer I'll get is 'you need more insulation', but be good to hear some thoughts. I'm wondering if my chimney is too wide. The whole oven is made of stuff we had lying around on the farm, and that was the right length of tube so I went with it.

There's pics of the entire project in here from start to finish - https://photos.app.goo.gl/ZgPi94eJJE6AJdY57

[UtahBeehiver]

The floor is not holding heat and you already know one of the reasons, insulation under the floor and on the dome are not the very effective. But it is what it is. The other main factors, the oven is potentially still wet (under floor and dome itself ) since you only built it this month as well as the sand under the floor is moist, this is the hardest moisture to remove once the floor is down. Last, the floor bricks, go all the to the brick support structure so all the bricks act like a big heat sink and pull the floor temp down. I do not think the chimney is an issue, better big than to small.