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Casting porosity differential diagnosis


Please excuse my apparent ignorance. I am self taught via books and
this resource. I have been successful with most, but not all, of my
castings, and recently I am having a string of problems that brings
me to your sage advice. The problem at present, after much trial and
error, is porosity.

I mix the investment (Silcast20) at about 180 grams to 80 ml of
water. I follow the burnout schedule closely, with a thermo that I
believe to be reasonably accurate, however it is not digitally
controlled. I cast into a flask that has been soaking at 900 degrees
for at least an hour. I use a centrifuge, and oxy-acetalyne, and
sling the metal once it appears liquid and shiny, adding a pinch of
borax just prior. I get smooth, totally filled castings, except that
inevitably there are several pinhole pores on most every piece. I am
on a tight budget, and have been using primarily re-melted silver not
pure casting grain, some scrap some remelted trees.

Is it the old metal, a factor I have not yet controlled in my

Is it the flask temp, or am I overheating the silver?

Is the investment too heavy (it is nearly double the recommended
mix, however the packaging mix resulted in very fragile stuff that
tended to break in to the pieces).

Should I just go ahead and weld up a vacuum chamber for my pump?
Still, I feel foolish not mastering a tried and true method before
taking on another process…

Thanks folks, R.


Hi Robert,

On the face of it sounds like it should work, on the face of it, but
if you’re pouring at 900’C (I assume it’s Celsius as 900 is way too
low for 'F) then it’s too high by a long ways, thick silver sections
should be around the 450’C mark (although going hotter won’t hurt)
and thin sectioned ones around the 650’C mark, above 750’C the gypsum
binder in the investment starts to break down releasing sulphur
dioxide, bad for the silver, bad for the elements in the kiln and bad
for you. BTW as the gypsum breaks down your molds get weaker.

The other culprit might be your sprues and gates; you might be
starving the cooling metal of liquid metal and so creating the

These are just some of the potential causes, I’m sure that other
readers will suggest other things to look at.

Cheers, Thomas.
Janstrom Designs.


you should never use more than 50% “old” or pre-melted silver. This
will cause porosity.



It is hard to tell without looking at the location of the porosity.
There are many possible causes of porosity but it is often said that
most porosity is caused by improper sprueing. In this case the
porosity will usually be in a specific location such as near the
point where the sprue is attached to the sprue if the sprue is too
small. Think about the metal cooling and think that the large masses
will solidify last and if they do not have molten metal to draw into
the casting you will get pits. This is called shrinkage porosity.
You would prefer that the shrinkage pits would be on “button” in the
pouring cup.

Most investments that I am familiar with recommend 100 parts by
weight of investment to 40 parts water. That would be 200 g
investment to 80 ml of water so your investment seems a bit thin.
Investment manufacturers state the the investment changes strength as
soon as you depart from their recommended mixing ratio. You could
have pieces breaking off.

That said, you have at least a couple of other potential sources of

  1. Old metal is hard to completely clean so a test would be to make a
    casting with new casting grain and see if that solves the problem.

  2. If you have sharp corners where the sprue is attached to the model
    it is possible for little bits of investment to break off and become
    embedded in the casting.

  3. I seldom use oxy-acetylene for casting silver because it is easy
    to over heat the metal. If you get the oxygen cone on the molten
    metal you might oxidize some of the metal and the oxides will mix
    with the pour.

These are all pure guesses. Companies such as United Precious Metals
have many pages of suggestions regarding the causes of porosity on
their internet site. Read that and see if any of the
possibilities give you any other ideas. There are so many possible
causes of porosity but improper sprueing is the first place to look.

One of the appendices of my book “Lost-Wax Casting: Old, New and
Inexpensive Methods” has a list of over 20 possible causes of
porosity. You have a sleuthing job ahead of you.

Hope this helps,

Is it the old metal, a factor I have not yet controlled in my

I can tell you with certainty that you need at least 1/2 new metal
in casting. I have reused old casting material and I save scraps from
my construction pieces (sheet) for new since it hasn’t been cast and
came from the refinery. It doesn’t matter wether it is new casting
"grain" or if it is scrap sheet, as long as it is new. That could be
a big part of your porousity. As for the other elements of casting
I’m sure someone here can help you, but that was just what I know!

Take care and good luck-
Stephanie Swanson



It could be a combination of reasons that are causing the symptoms
you’re seeing. I’d suggest looking at each thing one at a time
(controlling the variables) until you find the right combination for
your setup.

The things that raised “flags” for me in your posting were these:

  1. The flask doesn’t really need to “soak” at 900 for at least an
    hour. We tend to cast pretty much as soon as the oven temp reads 900,
    or within the next 1 - 2 hours. So the flask temp is still likely to
    be a bit higher when we cast. That’s not a problem at all and may
    result in better flow within the flask as the metal doesn’t freeze in
    thin places as easily.

  2. When you’re mixing the investment, the proper consistency is that
    of sour cream or yogurt (or a thin pancake batter). If you’re mixing
    much thicker than that, it could be causing problems. However, I
    wouldn’t expect to see pinholes so much as bubbles that couldn’t be
    vacuumed out or loss of detail on surfaces. But, it’s worth looking

  3. I also use mostly remelt silver, with the occasional coin or
    casting grain tossed in. However, you don’t say what TYPE of remelt
    you’re using… if chains, rings, etc., you could be getting a lot of
    solder contamination in there, which would cause pitting. Take a look
    at the scrap that you’re using and assess it for what’s likely
    soldered - cut that area out and cast without it (that can go to the
    refiner) and see if it helps.

  4. Try tossing in a bit of borax prior to the melt, as well as at
    the end before closing the lid on the caster. It will help bind the
    contaminants and will also prevent some of the oxidation as the metal
    is melting, which might give you a better surface finish.

  5. It doesn’t sound like you’re overheating the metal… if
    anything, you might be underheating it a bit. Keep in mind that
    metals have 3 temps to be concerned with: melt, flow, and boil. Once
    the metal reaches melt temp, it loses its crystalline structure and
    becomes a liquid (think slush). Flow temp is the “ideal” RANGE of
    temps at which the metal will flow into a mold. The higher in that
    flow temp range it is, the more liquid it is and the better it fills
    in all the little details (think water). Boil temp (the metal spits
    and bubbles) is something to be avoided completely (dangerous and
    doesn’t cast properly). But if you look at a chart, you’ll see that
    silver has a fairly large range between melt and boil that you can
    work with. Try letting it get a bit hotter and see if you’re getting
    better fill with it. It should look like very liquid mercury right
    before you cast.

Hope this helps!

Karen Goeller
No Limitations Designs
Hand-made, one-of-a-kind jewelry


Porosity is a casters’ mistress-- you have to treat her nicely, but
make sure she keeps her place. Oooh, did I say that (very un PC)!

The most effective means I know of to control porosity are to make
sure your sprues are the right size and placed correctly, to use
good metal, and to use your torch properly, avoiding prolonged or
over- heating and over-oygenating your flame, regardless of whether
you use a centrifuge or vacuum.

From what you describe, sounds more like flame/temp or metals issues
since you say the porosity is evenly distributed. Shrinkage porosity
will occur around sprues or in thick areas that are next to thinner

I know this will probably stir some people up, but oxy-acetylene is
a very hot and rather dirty flame for precious metals. Preferred for
the melt you describe would be propane or natural gas with oxygen–
cleaner burning gases, plenty hot. Hydrogen and oxygen is also
excellent for some applications, very clean, hot, but is trickier to
use and can be more dangerous-- uberflammable.

Very important-- Be sure you adjust your flame to a reducing
atmosphere (more fuel than oxygen). Preheating your crucible is also
a good practice.

My second suggestion would be to get away from just remelting old
metals and ALWAYS add 50% fresh metal, i.e. from the refiner, or
that you have alloyed from fresh metals yourself. The problem with
making your own alloys is quality control-- I would rather pay the
extra two cents and be assured of predictable results. Alloys are
more sensitive than you realize-- remelting old buttons without
adding fresh just perpetuates the problems and allows error to

Imhop, you would be wise to exchange all your old trees and metal
for brand new metal and start all over. You might try one of the
special silver casting alloys with built-in oxidizers and flow
enhancers-- most refiners have their own proprietary mixes, but we
find they are great.

Is your burnout cycle holding 1350 at the top end for at least one
hour? Stray gases from incomplete burnout can cause porosity.

Are you steam dewaxing? That can help the casting process greatly if
you’re using softer waxes (not hard carving waxes).

Doesn’t sound to me like your issues are related that much to
investing or to flask temp-- if you’re getting complete castings,
your flask temp is fine. Not familiar with your particular
investment, but if it’s not acting right with the manufacturers
recommended water/ powder ratios, you may have a bad batch-- this is
a common problem with investment. But that wouldn’t have much to do
with your porosity issues.

Jim Sweaney


The resons for porosity are many and mysterious, and several have
addressed that. Something else that was asked but not answered, that
I saw, was centrifuge vs vacuum… In my experience there’s no
advantage to either in experienced hands. Vacuum people swear by
vacuum and centrifugal people do the same. I’ve worked on many
castings by both methods and you really can’t tell the difference at
the bench - again, in skilled hands. Meaning that switching to vacuum
isn’t any magical way to solve porosity problems. The place where
vacuum takes off is in larger flasks, like 8" and larger. I have seen
an 8 foot long centrifuge, but when you get that large is when people
go vacuum, if not straight gravity pour.


This is an old post, but it’s a summary of responses to micro
pitting problems I was having;

First, as far a identifying the problem. The consensus seems to be
that what we have is shrinkage rather than gas pits.

It was said that gas pits tend to be more spherical and are usually
found throughout the casting, while shrinkage usually occurs in
heavy areas, has spongy pores that are really small tears.

Suggested remedies;

  • Sprue from thick to thin, your objective being that you want a
    homogeneous and smooth flow rate. Spueing from thin to thick will
    nearly always cause problems.

  • Keep your flask temperature for white gold as low as possible to
    get good fill with your machine (700-900 was suggested, but that
    depends on the type of machine).

  • Try to reuse less than 30% of white gold in castings. There were
    some good suggestions on how to reuse a greater percentage and how
    to super clean the buttons for reuse.

  • Regularly calibrate your kiln. Using ceramic cones is an
    inexpensive and reliable method.

  • Use a fairly substantial button with white gold.

  • Cast heavy and light weight white gold castings separately so you
    can adjust your flask temperature down further for the heavier
    castings and a little higher for the lighter ones.

  • Let the flask soak at the casting temperature for a couple of
    hours to insure that its at that desired temp inside the flask.

  • Keep you initial burnout stages long and at a fairly low
    temperature to soak the flask well and remove all of the wax like a
    dewaxer would do.

  • Keep you crucibles clean in an effort to remove any contaminates
    that inadvertently found there way there.

  • If possible, it’s better not to cast in your polishing area. It is
    a likely source of contaminates.

Goldsmithing is one of those jobs where the longer you do it and the
more you know, the more you realize how little you know.

Best regards to all,



I think your problem is the casting temperature and the metal.
Casting temperatures are dependant on the size and detail of the
piece and the type of metal being cast. A finer detailed piece
requires a higher casting temp. I looked at your website and would
guess from the pieces there that you are at least 100 degrees too
hot. Silver casts well as low as 750F. The metal that you are using
needs to be a least 50% new grain. The old metal should be extremely
clean and I like to cut it up small so that it melts at the same
rate as the new. Flux at 90% melt and cast as soon as you see the
metal shiny and round. Anything past this is overheating. I hope this
helps you out.

Shannon Calloway
Texas Institute of Jewelry Technology



It seems that this discussion about porosity is going on on at least
two lists right now. You might want to look at the Jewellery-Casting
group on yahoogroups. Several folks have pointed out the
importance of proper spruing to avoid shrinkage porosity which is the
most common casting problem.

It just happens that I have a sample chapter of my book that is on
spruing. Please log on to and then click on

The sample chapter is called “Placing Sprues” and is all about the
things that we have been discussing.

Feel free to ask me about any further questions that may not be
covered. Hope this helps,



Hi Robert,

This post may be a bit late as I’ve been in NY for the MJSA Show,
but I’d would like to help if I can.

Diagnosing the cause of porosity from a description can be difficult
because of the various types of it such as shrinkage porosity, gas
porosity, voids from spalling, etc. If you can take the closest
picture of the offending area you can and e-mail it to me, I would
gladly take a look and see if we can get to the bottom of it.

Shrinkage porosity is the most likely culprit. Is the porosity more
prevalent in thicker sections and get worse as you polish the piece?
Are the pits usually surrounded by cloudy areas or a haze? If so,
try gating the piece area a little more robustly and lower your
flask temp 100 degrees or so.

Good luck resolving your problem.

Casting House