In the past I have requested asssitance with the tendency of my gas
boiler ceramic pilot jet to become constricted with a hard dark
crystalline deposit. Well it has happend again and the pilot flame is
too weak to work the thermocouple.

I had prevoiusly been concerned about damage to the ceramic if I were to
scrape the deposit away. I now have a solution (literally). The black
gunge is dissolved by hot dilute sulphuric acid. A small amout in a
china mug, drop in the jet and place in the microwave. 15 seconds on
full power heats the acid whereas the amount of metal in the jet (being
very much smaller than the wavelength on the microwave) does not cause
any flashes and bangs.
A quick swirl round and rinse in lots of water yeilds a squeaky clean
jet.

I can now heat the water with the pilot alone (mild exaggeration
perhaps) and the thermocouple is once again happy.

Not a solution for the faint hearted but with sensible precautions (
like making sure SWMBO is not around!!) it does the job.

Oh yes - do wash the mug out before making a self congratulatory mug of
coffee!!

Regards

Bob