I don't know what a plastic ball from a yarn kit is, so I can't be of much help. What kind of plastic, are they rigid or is there any give to them? Maybe after I see the pics I'll have a better idea. I think silicone should work though.

Sounds interesting! Like gin_gin, I'd like to see a pic first. If they have some give when you push on them, might need to be stabilized first. I'm hoping to start an experiment this week-end on some plastic Christmas ornaments from the 90% off sale at Hobby Lobby (last January). Cut off the hanger part and enlarged the hole...gonna try to "inject" one with cement using a squeeze mustard bottle. Another I want to cover with mesh and a few layers of cement or thinset or grout (whatever I have leftover.....Either way, I'm hoping to end up with a solid surface to mosaic on...will let you know. THese dent pretty easily, so it may not work.

Pat

Good ideas Curb!! You and Deborah will hopefully keep us informed of the work in progress. Jane

Thought I'd let you know about an experience of mine - I found out that plastic EXPANDS in the heat. Of course, I live in extreme heat - Florida - but I see that you do, too (I'm on the other coast). I mosaiced on a plastic globe (from a light fixture). It was fine until it went outside and sat in direct sunlight in the summer temperatures. Then it cracked along the grout lines. My adhesive was thinset, and I grouted with sanded grout. The same thing happened on a resin garden ornament - it expands, too. It was okay on my lanai (in the shade), but not out in the direct sun. I've pretty much decided that anything that goes outside in my climate is going to have to be on terracotta or concrete.

Thanks everyone..They must be sold at Michaels or Joanns, they are in a knit kit, wrapped with yarn. They are kind of rigid but do have some give if you push hard. I think the reinforcing with something might be good, can't imagine injecting with concrete tho Pat, although even some would help weight it on one end, huh? Yah Lynda, FL heat is a killer, I will make sure NOT to put in full sun..I need to be more daring and just TRY it..I got a couple including a few cute smaller ones, like the small floating mirror ones in our koi pond. Will keep you posted! Thanks so much!
I know..Just DO IT!! lol AND finish grouting a bunch of other pieces AND the side slab, sheesh!;>)..pix to follow..
Debbie