Ink Wash is an incredibly old skool technique that I learned in High School. It was taught to us in Art 1 as a way to learn about value and tone, as well as their relationship. We used film canisters (retro) and India ink, which is waterproof once dried. The idea behind it is that you start with 3 to 5 levels of light to dark watered down ink. You apply it with a brush to capture the value of the image you are trying to capture. You add additional layers of watered down ink to achieve your dark darks. At the end you'd use a crow quill pen to add in details, or if you were "that good" you'd get a liner brush and use a light hand to add those final details.
In college I had a friend who did the most amazing ink washes in blue higgens waterproof ink. Amazing stuff. Since then I've sketched in black, red, blue, blue-black and a variety of other colors. Traditional is with black so that's what I've shown in my video.
I use Noodler's Black ink for this, but you can use any ink brand you want. The vials I use are from GouletPens.com. I used my syringe that I referenced in my last post about fountain pens to measure the ink.
Anyway, watch the video, try ink wash, it's an easy and fun technique.