The Kiln is just a kiln, nothing to worry about. Just joking, the kiln in Tap Smiths helps you smith rarer items, by increasing the max temperature every upgrade! You start with the Antique Wood Stove to smith your items, but after some upgrades, you can get up to the mighty Graviton Generator! (Level 12) The look of the kiln goes through various material changes, from bricks to clay, iron to steel, whatever, then it starts changing energy types, until you get to an energy ball.
Level 1: Antique Wood Stove
The appearance of this is very old fashioned, it has a pipe for ventilating steam out, and it's made of wood and metal, there is a fire blazing inside this costs 10 gold overall to level up from this.
Level 2: Old Furnace
It seems your kiln has been upgraded to a proper blacksmith furnace! The appearance is the same, but the top is rounded like a dome, and it's made completely of black metal. It has 2 upgrades so you have to upgrade it twice to get to the next kiln. The initial cost is 45 coins but then the overall price to upgrade is 105 coins.
Level 3: Pizza Oven
Looks like your good old smithery has turned into a pizza establishment! Your traditional furnace has been replaced with a pizza oven, it looks like an either a wood-fired or metal-made furnace, with a lot of fire, what is assumed is a timer, and a pizza! There are 5 upgrades, the first one costs 95 coins, the second one costs 120 coins, the third one costs 145 coins, the fourth one costs 170 coins, and the upgrade to Level 4 costs 195, overall the cost is 725 coins, a hefty fee for someone who might not have franchises, like I did.
Level 4: Clay Kiln
Now you're cooking with a real kiln. The clay kiln looks like a overall dome shape, with a stone base underneath the dome, a dome/base like entrance where a fire is blazing, a second square hole above that, and a ventilation hole at the top. There are 7 upgrades, the first one costs 220 coins, the second one costs 245 coins, the third one costs 270 coins, the fourth one costs 295 coins, the fifth one costs 320 coins, the sixth one costs 345 coins and the upgrade to level 5 costs 370 coins, overall the cost to get to level 5 is 2,065 coins, this may seem expensive, but if you dole out money to other upgrades, it won't go so slow.
Level 5: Brick Kiln
There isn't much to say, Clay to Grey Brick, it kinda looks like a fireplace in a Great Scottish home unit, it's overall a great looking piece. There are 9 upgrades, the first one costs 395 coins, the second one costs 420 coins, the third one costs 445 coins, the fourth one costs 470 coins, the fifth one costs 495 coins, the sixth one costs 520 coins, the seventh one costs 545 coins, the eighth one costs 570 coins, and the upgrade to Level 6 costs 595 coins. The overall cost is 4,455 coins, very expensive.