Carburizing flame
The carburizing flame, produced by burning an excess of acetylene, may be recognized by its three distinct colors. There is a bluish-white inner core, a white intermediate cone, and a light-blue outer flame. It may be recognized also by the feather at the tip of the inner cone. The degree of carburization can be judged by the length of the feather.

Neutral flame
The neutral flame does not alter the composition of the base metal to any great extent; therefore, it is the flame best suited for most metals. The neutral flame burns at approximately 5,850¡ÆF. A balanced mixture of one volume of oxygen and one volume of acetylene is supplied from the torch when the flame is adjusted to neutral.
The neutral flame is divided into two distinct zones. The inner zone consists of a white, clearly defined, round, smooth cone, 1/1 6 to 3/4 inch in length. The outer zone, made up of completely burned oxygen and acetylene, is blue with a purple tinge at the point and edges. A neutral flame melts metal without changing its properties, and it leaves the metal clear and clean. If the mixture of oxygen and acetylene is correct, the neutral flame allows the molten metal to flow smoothly, and few sparks are produced when welding most metals.

Oxidizing flame
The oxidizing flame is produced by burning an excess of oxygen. It has the general appearance of the neutral flame, but the inner cone is shorter, slightly pointed, and has a purplish tinge. This flame burns with a hissing sound. When welding ferrous metals, you can recognize an oxidizing flame by the numerous sparks that are thrown off as the metal melts and by the foam that forms on the surface.

Flame adjustment

To adjust the flame, light the torch by opening the torch acetylene valve one-fourth to one-half turn. With only the acetylene valve open, the flame will be yellow in color and give off smoke and soot.
Now open the torch oxygen valve slowly. The flame will gradually change in color from yellow to blue, and it will show the characteristics of the excess acetylene flame described earlier.
With most torches, there will be a slight excess of acetylene when the oxygen and acetylene valves are wide open and the recommended pressures are being used. Now close the acetylene valve on the torch slowly. You will notice that the secondary cone gets smaller until it finally disappears completely. Just at this point of complete disappearance, the neutral flame is formed.
To see the effect of an excess of oxygen, close the acetylene valve still further. A change will be noted, although it is by no means as sharply defined as that between the neutral and excess acetylene flames. The entire flame will decrease in size, and the inner cone will become much less sharply defined.
Because of the difficulty in making a distinction between the excess oxygen and neutral flames, an adjustment of the flame to neutral should always be made from the excess acetylene side. Always adjust the flame first so that it shows the secondary cone characteristic of excess acetylene; then, increase the flow of oxygen until this secondary cone just disappears.
During actual welding operations, where a neutral flame is essential, the flame should be checked occasionally to make certain it is neutral. This is accomplished by momentarily withdrawing the torch from the work and increasing the amount of acetylene until a distinctive feathery edge appears on the inner cone. Then, slowly decrease the amount of acetylene until a well-defined cone, characteristic of the neutral flame, is formed.
With each size of tip, a neutral, oxidizing, or carburizing flame can be obtained. It is also possible to obtain a ¡°harsh¡± or ¡°soft¡± flame by increasing or decreasing the pressure of both gases.
For most regulator settings, the gases are expelled from the torch tip at a relatively high velocity, and the flame is called ¡°harsh.¡± For some work it is desirable to have a ¡°soft¡± or low-velocity flame without a reduction in thermal output. This maybe achieved by using a larger tip and closing the needle valves until the neutral flame is quiet and steady. It is especially desirable to use a soft flame when welding aluminum, to avoid blowing holes in the metal when the puddle is formed.

Backfire and flashback

Improper handling of the torch may cause the flame to backfire or, in very rare cases, to flashback. A backfire is a momentary backward flow of the gases at the torch tip, causing the flame to go out. Sometimes the flame may immediately come on again, but a backfire is always accompanied by a snapping or popping noise. A backfire may be caused by touching the tip against the work, by overheating the tip, by operating the torch at other than recommended pressures, by a loose tip or head, or by dirt or slag in the end of the tip. A backfire is rarely dangerous, but the molten metal may be splattered when the flame pops.
A flashback is the burning of the gases within the torch, and it is dangerous. It is usually caused by loose connections, improper pressures, or overheating of the torch. A shrill hissing or squealing noise accompanies a flashback; and unless the gases are turned off immediately, the flame may burn back through the hose and regulators and cause great damage. The cause of a flashback should always be determined, and the trouble remedied before relighting the torch.