['< sod > alum solution has the property of dissolving steels while while not affecting aluminium alloy or [ [ base metals ] ], and can be again used to recover workpieces made in these metals along with broken toolbits < ref > https : / / at www. youtube. com / technical watch? v = fqzygreuywm ave demonstrates usage of alum to remove a broken stud from an aluminium engine head < / ref > lodged inside them. < ref > http : / / www. model - engineer. co. uk / forums / postings. asp? th = 91442 & p = 82 # posttop < / ref > as considerable expense and / or effort may have gone into machining a specialist part, but this can be a worthwhile exercise. { { citation needed | date = september 2016 } } < eod > < sod > alum solution has the property of dissolving steels while not affecting either aluminium or [ [ base metals ] ], or and can be again used to recover workpieces made in these metals with broken toolbits. < ref > https : / / www. youtube. com / watch? v = fqzygreuywm ave demonstrates usage of alum to remove a broken stud from an aluminium engine head < / ref > lodged inside them. < ref > http : / / www. model - engineer. co. uk / forums / postings. asp? th = 91442 & p = 82 # posttop < / ref > < eod >']