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Vacuum - yet again!


G’day folk; ‘Once more into the breach, dear
friends’… I’d like to offer the following comment and
suggestion: Firstly the atmosphere presses down (and around) at
the approx. average of 14.75 pounds per square inch at sea
level. Thus, when a vacuum gauge shows 29 inches (of mercury,)
there is approx. 0.49 pounds/ of atmospheric pressure left
inside the vessel. At 28 ins, there is approx. 0.98 lbs/sq in. in
the vessel. Is the difference worth worrying about when

The air removed from the vessel will contain moisture from the
atmosphere, etc- plenty of it in when de-bubbleing. I offer the
second suggestion that a simple trap containing recyclable silica
gel be interposed between the pump intake and the vacuum system.
whatever it is, in order to lengthen the life of the pump and
valves. And that the silica gel does get removed and heated to
remove it’s gathered moisture at very frequent intervals and so
recycled. Cheers, –

   / /
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(_______)  In sunny temperate Mapua NZ -

Autumn’s here…


I would like to nominate John Burgess for the “Orchid of the
Month” Award.

There are so many wonderful and talented Artisans on Orchid, I
would like to see us regognize them on a regular basis. “Orchid
of the Month”.

John Burgess consistently provides suggestion to explain or
alleviate problems. The sense of humor he imparts in every post
makes learning from him easy even when the text may be a bit

His knowledge is broad and covers a wide area of the Jewelry
making scene.

Bravo John



Hi John, In the many years that I have helped people in the
casting field,I have seen almost without fail that casting/
investing problems are related to the machinery as well as the
person doing the casting.The difference in Vac is certainly worth
worrying about. At 26 " merc, investment and water will not
"boil" at the investment manufacturers suggested water temp of
70- 72 oF … but raise the temp to 100+ oF(investment mfg co’s
do not recommend doing this) and the mix will boil fine , however
you will see different work times and a shortening of the life of
the pump unless meticulously maintained. All of the small
vac/cast systems i am familiar with will all pull better than 29"
merc when new.Mine is 27 years old and still pulls at that level
… it has had continuous oil changes ,custom built filters and
watertraps, testing and replacing of hoses,tubing etc.which is
why it works as well as it does.

As far as casting is concerned, the amount of vac also has an
effect and believe me when i say the closer you can be to 30"
merc, the better off you are.Vac gauges are notorious for not
telling the truth and must also be checked for accuracy when
problems occur. Those of us who do limited volume production
have seen the differences in vacuum casting when pumps are weak .
So, to answer your question, … yes, the difference is worth
worrying about.

Vive La Diference!!!
Daniel Grandi

I would like to nominate John Burgess for the "Orchid of the
Month" Award.

With all respect and thanks for the thought, I don’t really
believe this is at all necessary nor is it desirable.

Please bear in mind the John Burgess motto: Taure Excretum
Cerebrum Obsidet. You want a translation? Oh, OK: Taure (of a
bull) Excretum (obvious) Cerebrum (the brain) Obsidet (stands
in the way of; baffles) Sort it out for yourselves.
Cheers, –

       / /
      / /
     / /__|\
    (_______)  In sunny temperate Mapua NZ -

Autumn’s here…