[Jewelry making - Article 01066] - Re: [Orchid] Investment for detailed work

[Kenneth Ferrell]

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    I have to agree I have used R&R, Kerr satin cast 20 and Americast
    (which I won't do again) In New Mexico at 6800 feet alt. working
    between Albuquerque and Santa Fe. I had 7 people working for me and
    none were relegated to pulling bubbles off of cast pieces, I used the
    standard times, and mixed as called for as close as possible.
    Occasionally if casting very fine and small filigree pieces I would
    go to a 41 or 42 to 1 mix ratio but stayed with 39 or 40 to one for
    average size items, real heavy chunky men's rings and buckles at as
    low as 37 to 1 ratio. My castings were of high quality with few water
    separation lines or bubbles, 

    I've been using a Sargent Welch 5.5 CFM vacuum on a 12 in side draw
    bell jar and replaced my vacuum pad at least once a year, I also made
    sure my lines had no leaks , If memory serves me I would draw approx
    26.5 HG mercury I found that if you play with the mix you either get
    poor green or fired strength, I don't like dodging molten silver gold
    or brass, during this time we were casting average of 12 4 x 6
    flasks, some days we ran as many as 20 flasks, Altitude is a relative
    thing, in truth what is minus 3 atmospheres at an altitude of 6200
    ft, the mean altitude of Albuquerque, (which is over a mile high)for
    the benefit of the whiners in Denver, may only be represented as 25.5
    HB Mercury on a Gauge The same minus 3 atmospheres at sea level might
    be represented as 28.9 or 30 HG Mercury. Here at an Altitude of
    approx 600 feet above sea level my system draws 27.5 to as much as
    28.5 on days of low barometric pressure. In my opinion altering the
    mixing proportions given by the manufacturer is in the long run
    asking for trouble. Whether this comes from porosity caused by gas
    not being able to escape to impaired permeability, Investment
    cracking and throwing metal out the sides or end of the flask, Vacuum
    casting not being able to draw enough to sufficiently fill the
    cavities, or watermarks, 

    The only reason to have entrapped air bubbles on a casting is
    incorrect mixing technique, inefficient equipment (too small of a
    Vacuum pump or wrong type) or a leak in the system, 

    I recently started using Akron Investment from Akron rubber, at
    first it scared me because it seemed so thick when mixed to the
    stated proportions, I was delighted when I broke out the flasks, (No
    relation to the company but a satisfied customer) it is very unlikely
    that any two batches of investment will be the same viscosity when
    mixed any way since the quality and consistency of the raw materials
    will change from batch to batch. The viscosity is of secondary
    importance, the prime factor that manufacturers try and control with
    the most accuracy is the Work time. It has to remain as consistent as
    possible. So altering the water to investment ratio and the
    temperature might work, but why not just do it right and get better
    quality castings, this will lead to less rejects, castings that are
    of a higher quality and less work finishing. Unless of course you
    just like having to do extra work. The other thing to consider is
    that investment does not have a particularly long shelf life. It will
    absorb moisture every time the container is opened, I at one time
    considered building a dehumidified storage area for investment, or
    some way of vacuum sealing a container of aluminum or steel to keep
    the extra moisture from interacting with the mixing. 

Kenneth Ferrell
www.shadras.com 


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