Jack Aubert wrote:
>
>Separately using part of my brass, I made a base strip about 1" wide with
>rounded ends, and about 3/4 longer than the base of the gnomon. I drilled
>this with four small screw holes and brazed it perpendicular to the base of
>the gnomon. (Actually, I had it brazed. I had thought I could braze it
>myself, but didn't have the technique, or the right equipment, and had to
>take it to the metal shop where I had got the brass piece.) I drilled
>small holes into the slate and fastened the whole thing using little brass
>screws and plastic inserts, or possibly epoxy, I don't remember which.
Slate dials often avoided the need for a baseplate on the the gnomon by passing
it right through a slot. The gnomon was pulled into place with a tapered metal
pin through a hole behind the dial and perhaps secured with molten lead poured
in from behind for good measure. An angle-grinder with a narrow stonecutting
wheel would do much of the donkey work, again from behind, finishing with a
hacksaw blade from the visible side for a neat slot.
>
>Unfortunately, brazing of the brass shoe to the gnomon itself was the most
>expensive, and in some ways the most difficult, part of the whole
>operation. It required several phone calls (is it done yet?) to the metal
>shop, which lost track of what was for them a small nuisance job. I would
>like to learn to braze something like this, but think I need at least an
>oxy-acetlyne torch and possibly even an argon rig so this is now another
>one of those things that I plan to get into when I retire and have more
>time.
I would suggest you use jewellers' silver solder instead of brazing. This is
basically silver with copper added to vary the melting point. The 'Easy' grade
has the lowest melting point at between 605° and 723° Centigrade. When
soldering brass some copper from the parent metal will migrate into the silver
solder colouring it to a nearly invisible joint.
Secrets of success
1. The metal must be cleaned with an abrasive and no finger marks added to
contaminate the joint.
2. Build a 'nest' of firebrick around the job to contain and reflect the heat.
3. Apply the appropriate flux (obtainable from the solder source - or I can
give more detail) and place small pieces of solder in the flux ready to flow
into the joint.
4. Heat evenly and gently to drive off the water in the flux. If the boiling
action pushes the solder pieces (pallets) away from the joint push them back
into place with a steel wire.
The gas flame should contain extra carbon if possible to 'reduce' oxides
formed in the joint. Instead of combining with the metal to form copper oxides
it will combine with the carbon in the flame and pass off as carbon monoxide
i.e. the flame will actively 'clean' the joint. A carburising/reducing flame is
obtained by reducing the air or increasing the gas.
5. Heat as quickly as possible keeping the direct flame off the solder if
possible.
Prolonged heating will oxidise the joint and prevent the solder flowing.
6. The heat of the workpiece should melt the solder which will then flow
quickly into the hottest part. It can be 'pulled' into place with the flame.
7. Once flowed do not 'cook' the joint with further heating.
Your local library ought to have a book on beaten/hammered metalwork or
silversmithing which will give more information.
Hope this is some use.
Tony Moss.
