The short answer to your question is indeed longer firing times at
higher temperatures will result in much stronger pieces. PMC3 is the
strongest of the 3 versions, but only when full sintering takes
place at the highest temperatures.
I will only make rings of PMC that are thicker and heavier. Dainty
little rings won’t hold up to the wear and tear of regular use. I
may be alone with this opinion, but I don’t necessarily think fine
silver rings, in general, are the most practical because the high
abuse rings get and the softness of the metal.
I think a longer answer to your question may be more satisfying, but
a bit of history about Metal Clay may be needed, bringing light and
understanding to the brittle MC issue.
When Metal Clay (MC) was first marketed, it was marketed to the
hobbyist. Reason being is in Japan, where it was developed, they
have a majority of hobbyist and manufacturers. Unlike in the US,
where there is a much larger middle level of professional artisans.
Because MC does not make sense for mass manufacturing, they selected
the hobbyist for their target. Their aim at that time was to make a
product that had quick results – that meant low firing times. In
general, the philosophy was to provide a product that had "Perdy"
results quick and easy. Unfortunately, at the time longevity of the
product was not a focus.
As the products popularity grew and developed the pioneers and
artisan users of MC realized that the “professional artisan” out
there using this product in the US wanted more from it. They
demanded, not only beauty, but also stronger lasting results, and
new methods that would get them there. Among educated regular users,
longer firing times were explored as well as modified constructing
techniques that are best for MC vs regular silver smith work. They
realized all though you “could” fire at lower times, that didn’t
mean you “should.” So the general rule of thumb is to fire as high
as you can and as long as you can for the project with consideration
of gem and other inclusions.
My gripe with the manufacturers is that they still have not made
these basic concepts clear to the consumer. They still provide
firing schedules that put an emphasis on the lower firing schedule
verses the importance of the longer firing. Luckily, there are lots
of books and teachers, and websites that do discuss these concepts,
but I wish the manufacturers would catch up.
Education and experience is certainly the key to the success for
both you and your students.
I hope this helps.