Roll your own detector crystals

by Al Klase, N3FRQ

     It is possible to mount mineral samples for use in crystal detectors in the home workshop.  There have been persistent claims in the hobbyist press that this needs to be done using low-melting-point alloys such as woods metal.  However, my experience has been that specimens can be mounted in common solder if a little care is exercised.
 
    A simple mold can be fashioned from wood.  Secure a piece of dense hardwood, such as oak or maple, about a quarter of an inch thick.  Bore several clean half-inch holes through the wood.  Place the drilled piece on top of a second piece of the same stock, and c-clamp the pair to the edge of your workbench.  The goal is to eliminate all chance of spilling molten solder into your lap.
 
    With a piece of crystal and a pair of tweezers standing by,  fill one of the mold cavities with solder using a small hot iron or soldering gun.  Quickly position the crystal in the molten solder, holding it in place until the solder hardens.  The crystal will most likely try to float on the solder.
 
    I have found it best to use solid core solder for the bulk of the mount, and then add a little rosin-core solder on top to provide a small amount of flux.  Plumbers solder, both 50/50 and the modern lead free variety, has been used with good results.  If you use all rosin-core, the excess flux will bubble and boil and generally make a mess.

    After everything has cooled thoroughly, disassemble the mold.  You will probably have to force the mounted crystal out of the mold with a dowel.
 

Crystal Sources

 Usable detector crystals are surprisingly easy to procure.  The obvious sources are rock shops, and the gift shops in science museums.  Souvenir stores will often have local mineral samples, and even The Mall is a potential radio parts source: Check the Science Store.  Almost anything that is crystalline and appears metallic is a candidate.
 By far the best detector crystal is galena (lead sulfide).  Fools gold (iron pyrite) is another candidate, and actually seems better than galena for short waves.  Other usable minerals include chalcopyrite, bornite, cerussite, and molybdenite