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I want to tack fuse several dozen frit balls onto a blank in a random fashion. Do I need to secure each ball with glass tac (or elmers, etc) ? I don't want them to slide off the blank or slide into each other...can they do that???

You will find, if you pack them in real tight, that the glass balls expand when you introduce heat. Some that are very close the edge of your design may be pushed off. Leaving a tiny bit of wiggle room around the pieces close to the edge may prevent stray balls from escaping. Glass tac would work just fine. However I just discovered Bullseye Glasstac Gel and it worked really well for this type of scenario.

If your kiln is level, they shouldn't slide around. I don't ramp super fast so everything is flowing slowly. I use frit dots for eyeballs and tiny details and don't have trouble with them sliding around without glue.

Basically, if they can't stick in the spot you want without adhesive, they're going to move when the adhesive burns off. And if the schedule runs too fast, causing the glass to move, they are also more likely to slide around. So...gentle schedule, perfectly level position, and a layup that secures the frit balls exactly where you want them.

Also, of all the headache scenarios I've had when tack-fusing, a bunch of fritballs standing proud on flat glass is probably the biggest. In this old project, there was a lot of tack-fusing going on, and I did quite a few versions in clear that I tested for stress to make sure I had the schedule right. Adding the frit balls in the center sent the schedule's anneal time through the roof; as Studiodunn mentioned, you've got a lot of little glass balls all expanding and contracting independent of each other AND the base, setting up all kinds of stress point opportunities.

I'd therefore pay attention to the total thickness of the glass WITH the fritballs, and start making corrections for a very dimensional tack-fuse.
http://www.morganica.com/glass/fusing/t ... lotusbowl/