Gallic Hair

[woad&fangs]

Also, if the Celtic soap was close to it's modern derivate in properties, then it would have been quite easy to spike their hair with it

After searching around a bit it appears that you are correct. for you.

According to Pliny the Elder(courtesy of Wiki) the Romans produced a hair product out ash and tallow. Basically a crude soap. He also said that the Gallic and German men used it quite frequently.

Also, Soap apparently needs time to set after it is made. Before it solidified it could work very well for spiking hair.

Fat+Base= soap, The romans used tallow as the fat and ashes(NaCO3) as the base(look at the description of an Arrhenius base earlier in the thread). The Gauls probably used lime(CaOH2) as the base instead of ash.