Rendering black fire doesn't work in Cycles

Hi everyone,

I’m trying to render black fire in Cycles (blender 2.9). I can change the color emission with the ColorRamp in the shader editor, but when I try to turn it to black, the part of the flame which should be black disappear and let a smaller yellow/orange flame (see comparison with screenshots below).

Maybe the emission color cannot be turned to black but it still doesn’t work attributing the ColorRamp to the other color parameters in the Principled Volume box. So, how can I fix the problem ?

Here are the links to the screenshots.

BEFORE (magenta)

AFTER (black)

Thanks in advance for help !

PS: I’m new to the forum so unfortunately I can’t upload files here.

When it comes to volumetric emission, it’s better to think of colours as numbers.

For bright colours, the numbers are high and so your emission volume emits lots of light.

If you turn your colour to black, the numbers are zero, and so your emission is also zero. Zero emission volumes are transparent.

If you want part of your flame to appear black, you need to use volumetric absorption/scatter.

I haven’t tried it with volume emission - but for normal emission shaders, you can force cycles to emit negative light by making the emission value less than zero (e.g. -1). What will happen is that your object will appear black, and anywhere where this ‘negative light’ falls in your scene will appear darker too. It’s possible the same trick works with volumetric emission - maybe give it a try.

Thanks a lot for your help ! Indeed it makes sense about negative values, but I’m completely lost about how to fix it (beginner in volumetric rendering) :confused:

I don’t have any idea where to link the volume scatter or volume absorption boxes… I tried different stuffs (mix shader with an emission color set to -1, volume absorption set to -1, setting the emission strength of the Principled volume box to -1, etc) but it doesn’t work… What should I specifically change in my node setting in order to have a black flame ?

And what about this smaller yellow flame that stays fixed when I turn the emission color to black / 0 ? How can I change its color or delete it eventually ?

PS: Maybe I should have specified that my goal is to have a color gradient from white (base) to black (tip) along the flame, I don’t want it completely black nor that the negative light eats the white emission at the base of the flame.