CW wrote...
It would make a fine tool.

I agree that it could.

The brittleness is controlled by the tempering.

It can be to a large extent. In steel, the trade-off for decreasing the
brittleness by tempering is often lost wear resistance, and consequently,
edge retention. The optimal balance point for this trade-off depends on
the chemistry of the alloy as well as the application.

The extremely high carbon content of these steels and the lack of
toughening alloying elements to "glue" the hard micro-particles together
suggests that a solid tool might need to be tempered quite hot to reach
an acceptable level of toughness. That could have an excessively adverse
affect on edge retention, unless the very edge were tempered to a lesser
degree. (G)

A comparison between the No1A white steel and white cast iron is
interesting:

Alloying
Element No1A white steel white cast iron
============ ================== =================
Carbon 1.3-1.4% 1.8-3.6%
Silicon 0.1-0.2 0.5-1.9
Manganese 0.2-0.3 0.25-0.8
Phosphorus =0.025 0.06-0.2
Sulfur =0.004 0.06-0.2

(I hope the table isn't too messed up by a proportional font.)

The vast majority of steels contain less than one percent carbon, whereas
cast iron generally has more than two percent carbon. The No1A really
just falls on the steel side of the fuzzy area between steel and cast
iron.

The presence of silicon is especially intriguing because silicon affects
the solubility of carbon in austenite. My gut says that the silicon
content is too low to allow the the No1A to solidify with a substantially
eutectic structure, but it is suggestive nonetheless. It certainly helps
makes sense of the fact that Japanese woodworking edge tools often are
less tough than their western counterparts but exhibit better wear
resistance. That is, their failure modes tend to be chipping and
fracture, rather than abrasive erosion or (horrors!) plastic deformation.

Jim