What it the presoak used in this video?  And why is he doing it? 

 

I've seen this guys videos before and I always assumed it was kerosene. I think the idea is that it burns off using up any free oxygen in the forge before it can oxidise the steel, protecting it until its hot enough to apply flux or even allowing you to weld without flux.

Hopefully a more experienced smith will chime in and confirm this or tell me I'm talking rubbish.

MIght a soak in solvent also help get rid of the grease inside the cable?

Judging by how the forge flares up I'd say it's something flameable. Kerosene sounds about right to me, or some sort of degreaser.

Cable is usually oiled or in some cases greased, so it is probably for degreasing the wires deep inside.

2 hours ago, BIGGUNDOCTOR said:

Cable is usually oiled or in some cases greased, so it is probably for degreasing the wires deep inside.

He does the same process in a lot of his videos, including ones from freshly ground, stacked steel and even a billet made from screws welded together, which is why I think it's to prevent oxidation more than anything else. Whether it actually works is another matter but I can't think of another reason why you would soak a billet of clean steel in liquid before putting it in the forge.

With how rich it forge appears to be running after he put the billet in, my money is on kerosene. Especially if y'all have seen other videos by him using the same technique on stacked steel billets as there would be no reason to degrease freshly ground steel. Soaking a billet in kerosene as an alternative to the more traditional borax flux is a technique I have read about extensively on the ABS forums. One of the explainations I heard for this technique is that when the kerosene burns it coats the steel in a layer of carbon soot which prevents oxygen from coming in contact with the steel thus preventing the formation of scale. Which seems to make sense to me, though I am sure that there is someone that could provide a much better, more in depth explaination. While I am by no means a master at making Damascus, I have tried this technique on a few weld with good results. 

I guess it makes sense.  I thought if it was kerosene or something, inside the cable it might leave spots where it would not weld correctly, but if it burns off clean enough then good, and I can see kerosene doing this.  I got to give it a try.