Feeds:
Posts
Comments

Posts Tagged ‘meniscus’

YOU will have to learn a few words that may be new to you to differentiate struck from cast medals. One of these words is meniscus.  If you look up this word in a dictionary you will probably find an illustration of two tubes, one filled with water, one filled with mercury. The surface of the water is concave. The surface of mercury is convex – that is, bowed down at the edges from a higher center.

Cast metal is like mercury. Metal has the same physical property as mercury that makes it bow at the edges in unrestricted form. Cast metal cannot fill tiny cavities in a medal die design because the meniscus prevents the liquefied cast metal from flowing into these small corners. Thus it cannot reproduce sharp, crisp edges at the juncture of two surfaces.

The most obvious evidence of this can be found where the top of lettering meets the sides, another location is where the rise of relief meets the background or field of a coin or medal. With typical casting, the human eye perceives these edges and corners as “soft.” Under magnification, relief, particularly lettering, appears with “rounded” junctures.

Some medalists were skilled at purposefully modeling soft texture and soft lettering. Louis Roty in France was one. Victor Brenner in America was another. Medallists not as skilled may produce edges of relief that will look indistinct and amateurish.

Collectors, it should be noted, seem to prefer sharp, crisp edges because this is closest to the uncirculated state of a coin. Once wear begins, the sharp edges, like highpoints, disappear.

For a coin or medal struck by a die, the metal fills these junctures and tiny cavities by the force of the die during striking. Struck pieces therefore can have sharp, crisp corners and edges where the junctures of two surfaces may come to a point.

Diestruck pieces will appear sharp and not rounded if that characteristic is in the die. (But here, again, the roundness may be modeled into the pattern and reproduced in the die, or, the piece may have circulated and become worn.)

[The sharpness of the rim / edge juncture, as on proof coins, is something else. This comes from the amount of metal mass in the planchet and the amount of pressure applied, not from any modeling or anything in the pattern.]

A second term to learn is porosity. The surface of a cast piece may appear to have tiny pores or pockholes. These vary in size and are caused by dust or dirt in the mold or from trapped air.

When casting, skillful molders will blast the mold with compressed air to remove all contaminants just before pouring the metal. (Prior to this the mold may even be chemically cleaned.)

Trapped air bubbles prevent metal from filling all the nooks and crannies in the mold. This trapped gas tends to congregate around the base of relief or letters on the field or background. Skillful molders like to mold in a vacuum, or tip the mold and lightly tap it to let the air escape, or make elaborate vents for the air to flow out as the metal flows in.

Humorously, these pores or holes are called “craters” like bomb craters. (I say in my best Yosemite Sam cartoon voice imitation “That’s a figure of speech, Son. I say, that’s a hyperbole. That’s an overstatement, I say, to make them tiny holes look extreme!”)

If porosity is present on the piece you are examining, there is a strong chance the piece was cast. (“I say it was cast, Son. Take my word for it!” Enough Sam.)

In contrast to craters are nodules. These are raised lumps, also called bosses. (I won’t even crack a joke on that – make up your own lumpy boss remark!)

Nodules also result from dirt or trapped air occurring in a previous generation in the casting process. They formed when the mold was made. Now use that pockmarked mold and you get raised nodules where tiny craters were located.

Pieces struck from rusted dies will also exhibit nodules. Here again, these will congregate around the base of relief or letters. This is where moisture settled and attacked the iron metal when the die was stored. Prolonged exposure to a moist atmosphere creates rusting of unprotected steel dies. (Even storing dies one on top of the other with a struck piece between prevents rust, as well as other methods.)

Only with experience comes the ability to identify nodules from poor casting techniques versus nodules from rusted dies on struck pieces. I didn’t say this was going to be easy. Casting nodules are usually smooth; rust nodules are usually jagged and uneven.

Next term to learn: chased. It is so common for cast items to have porosity and nodules formed that they are most often CHASED – hand tooled to remove these tiny imperfections. The term is almost one word: cast and chased. (Can you say “castandchased” as one word?) In some cases you can still see the tool marks, even though the CHASER has a toolbox full of tools (like burnishers) to smooth the metal surface after any gross amount of metal is removed or moved around during chasing.

The busiest worker in any foundry is the CHASER – cleaning up after the cast piece is broken out from the mold. All those casting flaws should be removed and all surfaces smoothed where they are supposed to be smooth. For cast medals the trick is not to remove any DETAIL, to retain all the MODULATED RELIEF, keeping it intact.

The next term may be familiar. It is resonance. That is the sound a coin or medal makes when lightly tapped on its edge. The tone or clarity is caused by the metal alloy, absence of trapped air internally and its thickness. A cast piece will tend to have a lower-pitched sound, a “dull thud,” in comparison to a similar struck piece which produces more of a “ring” of greater clarity and higher pitch.

A ring test, however, is not exclusive or foolproof. A suspect piece should be compared with known specimens if you are testing for genuineness. Both struck and cast pieces will ring – after all, bells are made by casting. The only difference is the resonance. Have you got a good ear for pitch and tone?

A last term to be familiar with is a form of casting, but its results are sharper than even a struck piece. That term is electroforming. This is how numismatic electrotypes are made. These are made in an electrolytic tank where the metal is deposited on a pattern, one ion of metal at a time.

Thus electroforms are noted for their sharpness, plus their extreme fidelity to their pattern, far more so than diestruck pieces. This sharpness has to be in the model, obviously. Electroforming reproduces exactly the surface of the model. This is why electroforming – creating galvanos – were used for over a century for coin and medal patterns (and reduced on the die-engraving pantograph like the Janvier reducing machine).

Here are the three methods of producing a coin or medal and the sharpness of detailed relief each method can reproduce:

Casting

Detail reproducible down to a hundredth of an inch.

Striking

Detail reproducible down to a thousandth of an inch.

Electroforming

Detail reproducible down to the width of a molecule.

The following table summarizes criteria for visual inspection. If you have access to a lot of scientific equipment, you can examine the surface substructure of the metal or you can take microphoto- graphs. There you will see how different struck metal is from cast metal after it undergoes the stress of striking.

Struck vs. Cast Diagnostics by Inspection

Property Struck Cast
Relief edges and corner junctures Sharp, crisp, pointed Rounded.
Surface state Smooth, detail where intended, field generally smooth, all relief fully struck up. May have pores or background nodules present, may appear uneven;flat; detail looks “soft.”
Nodules present None; struck from rusted dies if at all. From the cast mold.
Multiple specimens Identical. Pores, nodules may vary in number, size, position on each cast.
Chasing tool marks None. May be present.
Ring test resonance High pitch. Duller tone.
Advertisements

Read Full Post »