As with much of the casting process burning out and casting lace is
a combination of variables.
Building up with a light wax can solve a couple of these:
-the wax can add thickness to the cross section, the effect of which
is a more easily filled mold cavity: less back pressure. With a thin
cross section, the air in the that part of the mold chamber, while
low in volume, must evacuate quickly (more quickly than a thicker x
sectioned mold chamber which takes relatively longer to fill) ahead
of the molten metal. This can result in non-filled areas
-the wax can also seal the absorbent surface of the lace which could
become water logged during investment or could negatively effect the
mold cavity surface as the lace wicks water form the liquid
investment, leaving a weaker, coarse or even crumbly inner surface.
-the wax can also act as a stiffener, keeping the lace from folding
and flowing as the heavy, wet investment engulfs it.
The negative of using wax, I have found is that it can obscure the
original intricacies of the lace. You can use a fabric stiffener like
Gak (sp?) to stiffen the lace and then spray it with a sealer like
Krylon. This solves the folding and absorbancy issues. But it still
leaves you with a thin x section…
When I used to cast crowns and bridges, I often made copings
(armatures/bases) upon which porcelain would be fired to produce a
life like crown. The thinnest parts of these wax copings were.2mm.
They cast fine. To compensate for the difficulties presented by the
thin sections, I often “blind” vented the mold cavity by adding a
length of wax wire to the wax model. This, when burned out, became a
tube into which air and gases could move from the larger mold
chamber, buying a little time and easing the back pressure.
I would also cast at a bit higher flask temperature, pulling the
flask from the kiln when it was hotter, which allowed the thin
sections more time to fill as the metal remained fluid just a bit
longer. The negative effects of casting at a high flask temp. --and
there are some-- were mostly negated by the relatively low volume and
mass of the casting.
The professional casters out there might find fault with my
reasoning, but this has been my understanding of the process and it
has worked for me in casting crowns and a lot of jewelry.
—one more thing, though… Casting the lace is only one part. You
still have a thin piece of metal. Something to consider.
Take care, Andy