Unfortunately it dries out, even when kept well capped and bagged.
I lost an entire 15g of silver solder like this a while ago and I've
got 2g of 9K yellow that's starting to stiffen up. Is there any way
to resoften it - such as dunking the end of the syringe in a
suitable solvent? And how can I stop it stiffening up in the first
I don’t know if this’ll work for you, but here are 2 schemes that
work for me. I’ve never done a chemical analysis on the paste solder
that’s sold in the US, but I suspect the components are granulated
solder, flux (some include fluorides) and a liquid vehicle (probably
a mineral oil of some type or glycerine). The paste solder I use
comes in a syringe, 1 dwt for gold & 1/2 oz for sterling.
The simplest way to soften stiff paste solder is to soak a cloth in
hot water. Wring out most of the water & wrap the part of the syringe
containing the solder in the cloth. The needle can be on or off for
this procedure. If the needle is on, you may see a little bit of
liquid ooze from the needle.
Another way is to warm the syringe under a lamp with about a 60 watt
light bulb. This approach has a down side. If the lamp is placed to
close to the syringe & left too long, it’s possible to melt the
plastic syringe (voice of experience).
If you use paste a lot, building a ‘warmer’ is the best approach.
I’ve built one that uses a 4 inch (100mm) square by 2 inch (50 mm)
deep covered box. In use, the box is stood on 1 of the 2 inch sides.
Two holes are placed toward the top of one of the 2 inch sides. A 5
watt light bulb is located near the bottom of the other side. A wire
bracket maintains the tip end of the syringes above the light bulb.
The syringes are placed in the box diagonally from the holes to the
Turning the warmer on about 5 minutes before using the paste seems to
work for me. With the 2 holes, 2 different temperatures of paste can
be kept ready for use. I’ve used this for paste that was about 6
months old with no difficulty.