Abdullah,

Like most highly reflective metals, I imagine that molten tin (which is the dominant component of modern solders) has a rather small emissivity.

Let's Google:

Molten copper:

http://iopscience.iop.org/article/10.1088/0957-0233/24/1/015603/meta

Molten copper is about 0.08

Bright solid tin:

https://www.mastercool.com/media/emissivity-metals.pdf

0.05 at 100degC.

Add some flux, some dross, and emissivity might reach 0.1.

Surely heat loss from radiation is far far smaller than conduction?

Yes. Thank you so much for your informative reply.