what is the fire of circulation?

Ignis circulatorius - distillation by heat evaporation causing steam and condensation in a vessel

http://www.hermetik.ch/eidolon/lexikon/lexikon_a-z...

The above website is German, as is the source for the symbols you linked, the 1755 Medizinisch Chymisch Alchemistisches Oraculum

Edit:

The triangle represents fire, the c or cresent may represent purify.

In alchemy, circulation is continuous distillation of an element or compound in the same vessel.

So "fire of circulation" could also be "purifying fire" or a process changing an element using fire.

The crescent moon or Luna is associated with silver and magic.

http://en.wikipedia.org/wiki/Astrology_and_alchemy

http://en.wikipedia.org/wiki/Argent

Calcination is a alchemical process meaning to purify by fire:

http://www.alchemylab.com/calcination.htm

The c may also be calx: http://www.fileformat.info/info/unicode/char/1f74c...

"Calcination, which works with the element Fire to burn away dross and reveal hidden essences. The word “Calcination” (and such related words as “calcify” and “calcium”) are from the Latin root calx, which means limestone or bone. To calcine something is to burn it until it turns chalky white, reduce it to ashes, or cremate it. After Calcination, the substance at hand is no longer affected by common fire."

http://azothalchemy.org/azoth_ritual.htm

Edit 2: It is likely the meaning is inconsequential, the animators just liked the look of the different alchemical symbols : http://mf99k.deviantart.com/art/Gravity-Falls-Symb...

Edit 3: "Fire of Circulation" is also a literal translation of the French term for "Traffic Light"

http://fr.wikipedia.org/wiki/Feu_de_circulation

Edit 4: I think the image you linked may be a mistranslation, as the triangle with a complete circle around it is labeled "fire of rotation" as a literal translation for french feu de Roue from the Latin Ignis Rota which is actually translated as "fire wheel" and also represents coral http://www.symbols.com/encyclopedia/30/3022.html

The Ignis rota symbol can be seen in the works of Basil Valentine from 1670

http://www.meta-synthesis.com/webbook/35_pt/pt_dat...

Meaning "fire surrounding the matter to be fused in a vessel"

Alchemical Symbol For Fire