The article presents welding imperfections, particularly the most dangerous of them, i.e. cracks, tested using the penetrant method. The tests involved making natural cracks in aluminium alloy AlSi1MgMn. Afterwards, the width of cracks and the profile of roughness were measured. The investigation also involved a series of penetrant tests performed in order to observe how a given factor (crack width, crack surface roughness) affects development times in penetrant tests involving joints made of aluminium and its alloys. The article also presents the determined development time recommended for aluminium and its alloys in order to detect indications originated in cracks as well as analyses penetration times in penetrant tests.