Thanks for the advice. I found something interesting.
Blue-Tack is a fascinating substance that belongs to a group of materials called Bingham Plastics.
Bingham plastics behave strangely in that they behave as a rigid shape at low stress but can flow in the same way as a liquid at high stress. Blue-Tack is a Bingham plastic because it flows and moulds when pressure is applied but sets when you stop moulding it.
No one knows the exact composition of Blue-Tack as its a trade secret, but some of the substances it contains have been revealed.
One of its main components is a very sticky synthetic polymer of secret composition. This is what makes Blue-Tack so sticky. However, the polymer is so sticky, that it would make the stuff unworkable if this was the only substance Blue-Tack contained - you wouldn't be able to remove it from your fingers for a start.
To reduce the stickiness, naturally occuring minerals are added to the polymer along with various natural mineral oils. The interesting thing here is that before the addition of these adulterating substances, Blue-Tack is classified as a very viscous liquid.
Without this filler/oil combination Blue-Tack would flow like an extremely slow moving liquid, making it virtually useless for sticking posters etc on walls and when I say slow, I mean slow - it could take weeks to travel a few inches on a vertical surface.
It is the mineral oil that leaves the residue on your fingers after handling and it is the oil combined with the minerals that would cause the sparking in the microwave oven. It seems that some of these fillers are metallic in origin and the effect you saw is the same as if you placed a piece of metal in the oven and switched the microwave on - not something to be recommended!