The first time I tried setting up an infrared flash to do flash photography without visible light was back in the film days. The experiment didn't last long due to the price of materials. To filter the flash, I saved up and bought a 3x3 Kodak 87C infrared filter. These things aren't cheep. I think they were like $45-50 back then. Now they go for about $57. One thing to keep in mind with a filter that blocks all the visible light and only lets infrared pass is that the visible energy turns into heat.
The first time I popped the strobe after taping the flash to the front I had it at full power and the filter singed, hissed and crumpled up a fair amount. If you have ever seen a piece of surran wrap get to hot and crinkle up it's the same thing. This was when I was in high school or early college. And while $50 is nothing to sneeze at today, it was way more of my income back then.
This time I'm going to try the trick of using an unexposed but developed piece of slide film for the filter. Originally, I was thinking about doing this with my Alien Bees which would have probably required getting 4x5 film, but since I've been reading a lot of strobist, I'm going to be running this with smaller strobes which will allow me to run with 120 film. If this works, it'll be waaaayyy less expensive per filter, and I won't mind as much if they burn themselves up.