See also Erosion Tips and Tricks
Linear Burn: Linear Burn is just a subtract with a 1-x on the bottom input. Linear Burn will become dark overall as it fades. If you want the gradient to be a consistent length, go with Linear Burn.
Color Burn: Color Burn is the same but it divides the result by the inverse of whatever was subtracted. Color Burn will always keep white values white, so you keep your value range, but the length of the gradient will shorten as it fades, resulting in sharper edges.
What I like to do is use a Linear Burn, then pass the inverse of that into the emissive, so I get a hot glow on the edge of the crawl that fades as the alpha gets more opaque.
One trick I figured out, in order to fade from opaque to completely gone by lerping 1-0, I multiply the length of my gradient by the lerp alpha and add it to the lerp alpha. This way, a lerp alpha of 1 will always paint the whole mesh white, instead of leaving a little bit of gradient at the end.
Another trick I figured out is to use “equalized” noise textures for my alpha crawls. In Photoshop, go to Image > Adjustments > Equalize. This will make it so an alpha of 0.1 will reveal 10% of the texels, 0.2 will reveal 20%, and 0.9 will reveal 90%, etc. Otherwise, your lerp will have more of an ease-in-ease-out effect, which I’d rather do in the engine if I want it.
Linear Burn = dest + src - 1
Color Burn = 1 - (1 - dest) / src
target = destination (bottom layer) blend = source (top layer)