'Franklin Hearth' change process- Hearth devised at first was a shape that simply put 2 steel plates in a diagonal direction (left), and later made the steel plate move (right) so that made it play a heat proof role.

Fire and smoke in the hearth was a one body. People pondered about how can the smoke be sent and can the heat be held. An architect named 'Louis Sabo' produced an idea around 1640.
It was that raised the hearth floor by 10cm extent and made a space leaving a gap between the floor and the fire rear side wall.
It is because why, if so, air in that empty space was warmed up and subsequently heat loss through the hearth floor was reduced.
If we think now, we see that it is nothing particular, but it was an innovation in those days.

Benjamin Franklin who is a meritorious retainer at the founding of USA and renowned for inventing a lightning rod began to remodel the hearth whole by himself around 1740 judging the hearth is neither warm nor good to health.
He installed 2 steel plates in a diagonal direction to 15cm extent so that made the flame and smoke enter into this gap.
By doing like this it was able to reduce a draft coming into the chimney and decrease the smoke too. Franklin developed this system so that changed to make the steel plate hang up vertically and move.
By doing like this it was able to control volume of the draft and subsequently it was able to play a heatproof plate role.

Franklin produced a third idea again. After making a groove vertically on the both sides inner wall in front of the hearth and inserting the steel plate therein, controlled burning speed and heating power to an extent that push this steel plate upward or downward.
Furthermore he made an air inlet that is helpful for combustion in frontal part of the hearth and got out of several bricks under the floor so that prepared a passage that can make a fresh air come into the inner empty space of the hearth then warm up.
This is the very 'Franklin Hearth'.(Picture 3)