The cracks around the wire imbedded in the glass in the above image are not incompatibility cracks. They do not surround the square piece that traps the wire into the glass. These are from differential expansion/contraction stress between the wire and the glass.
Picture credit: Charmaine Maw |
Kanthal and nichrome wires are best as included wire hangers. They are designed for high temperature work and so do not weaken from the heat. This means that high temperature wire as thin as 0.5mm/22 gauge can hold a lot of weight. Much greater weight than is used in most glass objects to be hung rather than fixed.
Profile
A sharp tacked piece needs to be fired
as though thicker. This example is a single layer base and a square of glass to
trap the wire fired to a sharp tack. It needs to be fired as though 2.5 times the thickest part -
15mm. A rounded tack fuse of the same layup would need to be fired as for
12mm.
Layup
The use of wire in glass needs to
consider how the air will escape from around the wire. Yes, if the wire exits the glass, there is a
channel for it to dissipate. But air tends to collect along the length of the wire. If the wire
is fully enclosed in the glass, the layup must accommodate the need for air
escape routes. This might be with a fine
layer of powder, design elements, chips of glass to hold the outer edges of the
glass up for longer, or other devices.
Scheduling
The example shown at the start of this blog, is a sharp tack and needed the 2.5 times scheduling. That probably would have avoided the crack in the single layer base. That single layer cools faster than the wire with the added piece of glass. A bubble squeeze is a good idea, even though it would not normally be considered. This gives the best chance of reducing the bubbles that form around the inclusion.
You need to be careful about
increasing the ramp rate until the glass has passed out of the brittle
phase. This is about 540˚C/1005˚F. The
increase in the ramp rate during the brittle phase may cause cracks. It is, of
course, more likely to occur during cooling because the metal will be
contracting more than the glass during the brittle phase. This contrast in contraction rates induces
stress that may be great enough to crack or break the glass.
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