I like other posters always pack my pipe just shy of the rim. However during the smoke I do sometimes notice a shiny dark patch (tar) slowly appearing on the rim. I believe it to be carried there by the smoke itself as it wends its way out of the bowl. Once the bowl is finished I immediately remove the stain with good old spit but if left to harden, over time you will amass huge amounts of this 'lava' that will prove to be a real pig to shift so best to stay on top of it!
Regards,
Jay.
Regards,
Jay.