Absorption of Hydrogen, Oxygen, Nitrogen, or Contaminants. If I had to guess looking at it, I would say not enough shielding gas in the puddle. What cup did you use? I like glass cups; it lets you see the puddle better and also gives a little extra time for the gas to hit the puddle. The metal not being clean can also cause it, but I think unlikely in this case. The other thing that comes to mind with the gas is if your flow wasn't high enough then it will cause the above issue, almost the same if the gas blows away too soon. Thats what the cup is for.
Everything that Ranger has said above is spot on. On your tig machine depending on $ quality of it you can adjust the pre and post gas flow. Just the time the gas flows to the torch cup before and after to shield and cool the weld. More expensive machines have a foot pedal to really fine tune your amps Sometimes applying hydrofluoric acid pickle paste to pre clean the metal can really help too but that is super nasty stuff designed for dissolving bodies and anything else you can imagine ,so be real careful if you start playing with this stuff. Eat your eyeball out in 2 seconds. Playing around with 100 year old oil soaked metal is not easy and you did real good job.
