I use these balls in biofilters. Washing is good, but the best thing to do is to stick them in the microwave when still wet.
The idea is to overheat the water in the micropores which will then turn into hi-power steam expelling all the waste out.
BUT . . BE CAREFUL! They can overheat VERY QUICKLY.
Start at just a few seconds at low power, just to see what happens. Then increase time & power untill the balls start to froth.
That's it!

it could be salts which may cause your Ph to fluctuate. it may be a calcium buildup (aka scale) which I do not believe will hurt your plants.

thanks for the replys, Microwave sounds good but not sure what the wife will think haha, might have to look at buying a cheap microwave for the shed. I do get a bit of ph fluctuation usually it needs lowering.

Oh, it's less than dirty dishes, and I only do this once a year. Also, you can't put a lot in the oven. Not recommended!
I wash the balls with river sand, then place them in a circle on a pyrex dish. 10 seconds on hi. Into a bucket with water, followed by the next lot.

I use clearex once a month to remove mineral salts from the clay pebbles and baskets. I also boil them before using them again in plain water to sterilize them.