We were going to start making wooden frame for the second chamber and finish up arch first, but we decided to work on the third chamber exit flue and also 2nd and 3rd chamber side walls before that. Just because we thought exit flue walls and side walls had better be connected in order to make safer for arch.... very hard to explain though.
We mostly built the third chamber exit flue part with mortal this afternoon. Then laid brick for the side walls temporary without mortal and see how we can play with this huge puzzle. Even you draw perfect design on the map, but always something to be changed.