So tell me this: don't all forges create hot spots?
The short answer is yes, however, it's a matter of degrees, literally. In general forges of a square or rectangle design, suffer the most from "hot spots". This is because the burner(s) cannot help but be directed into the forge at 90 degree angles, greatly increasing the problem of hot/cold areas (spots). There is a tremendous difference in the size and intensity of the hot/cold spots within a circular forge versus a square or rectangular design. With careful consideration of burner placement/angle, hot/cold spots can effectively be eliminated within a circular designed forge.
What are the bad things about hot spots?
When it comes to forging "straight" steels, intense "hot spots" can create havoc with heat sensitive steels such as 52100. This particular steel is very heat sensitive during the forging process. It will give you no outward indicators that it's been "over heated"....until you finish a blade... then it will cut no better then a piece of sharpened mild steel. I've had and seen various forged 52100 blades that would either simple be terrible cutters, or worse, have portions of the finished blade that would cut as I would expect 52100 to, and other portions of the blade that were terrible. Experience has taught me that usually a blade with "good" and "bad" portions was usually forged in a multiple burner forge of square/rectangular design. With other steels, there are a number of detrimental consequences that can occur because of hot/cold spots, which manifest themselves in various ways..... an example of this are blades that warp badly in the quench....most don't realize it, but this is often a result of uneven heat during the forging process and/or the uneven heating prior to quenching, or attempting to conduct various thermal cycling procedures with a forge that exhibits excessive hot/cold spots.
Usually by the time an individual is ready to delve into making damascus (laminated steel), they have discovered/learned how badly hot/cold spots can effect things, but there are those who are impatient, and dive into it before they are ready, with a forge that is ill suited. The results are generally catastrophic in failed welds, warped or cracked blades, etc.
And how could a forge not create a hot spot or two?
It's all in the design of the forge. There's a reason that most seasoned Bladesmiths build/choose to use a forge that is of a circular design, with a single burner. These types/designs create the least amount of "hot spots", and with careful consideration of burner placement, hot spots can be minimized, if not eliminated. The main reason you see most forges that are offered for sale being of a square/rectangular design, is because that particular design is easy/cheap to build. The consideration of how effective or ineffective the forge works is rarely a consideration.
When I designed/built this welding forge, the elimination of hot spots was foremost on my mind:
http://knifedogs.com/showthread.php?43590-New-Welding-Forge-WIP
I managed to accomplish the goal. Although this forge is large/heavy, and it takes an hour or more to reach/level out to welding temp, there are no "hot spots" within the area that billets can be introduced. (there's always a level of "give-n-take" when it comes to forges.... it's up to the individual to decide what aspects are most important/desirable, and what aspects (on the down side) are acceptable.....and then decide the best way to gain those aspects they most desire, while minimizing those aspects they do not want.)