Once you progress with the game you have to build and use smelters.
If you have build a smelter in the 3D world you can access it by stepping in front of it and hit the "use"-key.
This will open the Smelter screen :
Section(1) is the smelter control. Section(2) is your backpack. You can interact with items via drag & drop.
The smelter control
Section(1) is the fuel control, here you can put up to two materials to heat the smelter. You can see that there is charcoal in chamber one.
The green bar indicates the burn-time of one unit charcoal - the red/orange bar indicates the min. and max. temperature the charcoal will produce.
The little white lines show you the amount of time and heat you need to smelter one unit of the item you want to smelt.
Section(2) is the oven control. The bar with the hourglass shows the time the fuel unit will burn until a new unit is needed.
With the oven handle you can throttle the air flow through the smelter. The bar with the fire and thermometer shows you the actual heat in the oven.
The two chambers on the right are for the source items, you can combine up to two items to smelt.(for example - sand and chemicals will melt to glass)
Section(3) You can start the smelting process with the upper button, or extinguish the fire with the lower button.(the fuel item is then lost)
Section(4) The Bar with the percent sign shows you the actual smelting progress. In the chambers on the right you will get the result item of the smelting progress.
The lower chamber is for the ash.
If you start the smelter process you have to adjust the airflow to set the temperature right.
The oven throttle
When the fire burns you maybe have to set the right air-flow to generate the right temperature for the smelting process.
If you raise/lower the throttle the heat-bar will show you the actual amount of heat in the oven.
If you let the smelter burn with full air flow you can generate the max amount of heat but the fuel item will burn-out much faster.
Adjust the temperature until the little fire symbol is green.
Once the process is set up properly you can walk away and do other things.
The smelter will run until there are no more fuel or source items. The process also stops when the ash pan or output chamber is full.
A VIDEO TUTORIAL FOR USING THE SMELTER IS AVAILABLE IN THE VIDEO-TUTORIAL SECTION.