[GSBN] Traditional burnt sand mastic recipe & experiences

[andy at ecodesignarchitects.co.za]

Hi good folks
has anyone got experience with making up one’s own traditional burnt sand mastic for caulking around openings and for providing a flexible seal for expansion jointing between different materials.


Down South here in South Africa we only get conventional silicone or polyurethane type joint sealers/caulking agents, which are environmentally dodgy and unsuitable for natural finishes.  I gather that people in the UK and US are using a traditional burnt sand mastic with certain suppliers selling it as a product.

As far as I can tell, you can make your own by roasting or baking a fine sharp sand to sterilize and drive off all moisture and then about an hour before use one blends it with a high quality boiled or double boiled linseed oil. It seems some caulk may also be added to the sand mix, but I am not sure if this is required or not. 
I was also wondering if you could sun-cure a raw linseed oil rather than use the boiled type in the same way one sun-cures linseed oil for an earthen floor.

Can anyone advise?
It would be interesting to hear people’s experiences. 



Many thanks 

Best regards
Andy Horn

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