Tar is viscous and sticky before hardening with a glaze like appearance, Stage 3 Tar is a highly concentrated fuel which can easily catch fire resulting in a potentially devastating chimney fire. You can find out more about the different stages of soot right here
Stage 1 & 2 soot can easily be removed by brushing and using power sweeping, Stage 3 can only be removed by being chemically treated.
I use something called “Cre-Away” & is a four component powder that safely modifies glazed stage 3 creosote.
When applied Cre-away will react with the tar and chemically modify it changing into a substance that can be more easily removed.
I will preheat the chimney and apply Cre-Away using a small compressor. The flue will be fully coated and left for 2 to 4 weeks producing a product that can then be removed by sweeping. I will then issue you with a current chimney sweeping certificate for your insurance.