The composition of standard IT solder is by weight:- fine silver
80%, copper 16%, zinc 4%.
Notice that the IT solder does have the zinc as stated in the
formula above; it is not zinc free.
It is not prudent, no matter how well you solder a seam, to have any
zinc come in contact with the enamel. When the enamel hits the zinc,
or sometimes just near it, it can cause the enamel to pit, crack
and/or chip. This chemical reaction of the vitreous enamel to the
zinc may not happen immediately, it can even happen down the road.
So, to be safe, choose to use eutectic solder. The eutectic solder
contains only silver and copper in the correct proportions to make
flow at a temperature lower than either of the two metals will flow
if melted separately. One thing to note about the eutectic is that
if you are working on copper, you can get a silver shine on the metal
and also the reverse, when using this on silver, you may have a
copper shine. To eliminate this if you are using clear or transparent
enamels, simply gently abrade off the “offending” color or if silver,
depletion guild to have a completely silver surface. If you are using
opaque colors, there is no reason to abrade if you soldered well and
created a smooth area and it meets with your standard of
craftsmanship; the enamel will be fine over the eutectic solder with
out pitting, chipping, cracking or breaking if you have done all the
other processes correctly and your metal is thick enough to prevent
any torque in the piece.
Linda Darty in her book “The Art of Enameling”, c 2004 Linda Darty,
Published by Lark Books, has many comments on soldering and enamels.
I will quote from page 55 in section Hot tips- Soldering With
Keep in mind that the enamel will pit or discolor if it is
applied over hard or IT solder because they both contain zinc. I
always use eutectic solder that is made of only copper and
silver, if I plan to enamel over the solder joint. If using
transparent colors over eutectic solder on silver, it should be
depletion gilded until the surface is bright silver before