Success, sort of.

A second three hour high-heat smelting reduced the contents of both crucibles completely. The copper came out as a single large button. The tin was different. There was one small button (5g) and a three layer "cake" of slag... a light grey very granular layer was the bulk of it. Atop that was a dark grey layer and atop that were little balls of tin, like a froth, embedded in the borax.

In researching the tin, I find that tin forms a significant amount of tin silicate slag, given the chance. Historically quicklime was added to displace the tin from the silicon. I will try that next time. 40% of the mass of the cassiterite in quicklime is the proportion recommended.

2013 begins: Smelting in a kiln

I got a Cress kiln a while back. I refitted it with a (much, much) longer cord, and here's what it looks like now:

I tried smelting copper and tin in it today, using crushed charcoal as a reducing agent and borax as a flux and sealant.

Crucible 1:

100g cassiterite
 10g powdered charcoal
  5g borax

Crucible 2:

 70g malachite
 10g powdered charcoal
  5g borax

Both crucibles were covered by ceramic discs that fit loosely.

I heated the kiln on "low" for about 30 minutes, "medium" for about 30 minutes, and "high" (dark orange heat) for 2 hours, then reversed the process for cooldown, for a total of about a 4 hour total firing. When I pulled them out, imagine my surprise finding only partially decomposed ore, and plenty of charcoal left. The copper was considerably further along than the tin in terms of decomposition, but neither had a substantial lode of purified metal.

Tomorrow, I shall extend the time for full heat from 2h to 3h and see what's what.