The Seared Tank is a block added by Tinkers' Construct that holds the lava which fuels the Smeltery, and also holds any other molten alloy. It can hold four buckets of fluid, and will send lava directly to the Smeltery Controller.
To add a fluid, simply hold a bucket of molten metal or lava and right-click the tank itself. Once the tank is full, right-clicking will continue to result in the bucket "swatting" at the tank, but your bucket will remain full. By using a seared faucet you can directly pour the desired molten alloy into any tank that is placed below the spout, so long as it is does not contain any other molten alloy, and the tank isn't full. The benefits of using the tanks as storage are great, as it allows one to remove small amounts of a fluid to be processed at a later time, freeing up the Smeltery to make new alloys
It is also possible to add seared spouts directly to the tank, which will pour the fluid into the desired Smeltery block.
Adding more than one Seared Tank to the Smeltery will not speed up the smelting process. The smeltery will simply use the lava from one tank, then move on to the next. This can be useful if you plan on smelting large amounts of ore.
Be careful not to right-click the ground in front of the Seared Tank, as this will dump the lava on the ground in front of you.