What is Porosity? In the simplest of terms, porosity is how holey a material is. No – not “holy” in a religious sense, but “holey,” as in “full of holes.”
Materials such as pumice stone and concrete tend to be very porous, while materials like glass and hard plastic are relatively non-porous. All materials have some degree of porosity, even if the pores/holes are not visible to the naked eye.
When it comes to jewelry, however, those little holes can become a big problem.
Porosity Bubbles, Gas, and Microscopic Shenanigans
The metals used in jewelry tend to be relatively non-porous and contain only microscopic holes. These microscopic holes, however, can become not-so-microscopic when gas is absorbed into the metal (typically during a heating process) and then released incompletely from it as the metal cools.
While the small dimples on the surface of a jewelry piece can look ugly, the real danger of porosity comes from the structural damage the bubbles cause as they expand beneath the surface. These bubbles weaken the structure of the ring and can lead to cracks, breaks, and discolorations.
Porosity in jewelry can be like an iceberg. I once went to polish out a little porosity hole. And I polished and polished. And the hole got bigger. As the wheel spun down as I turned it off, I looked at the ring I'd been so excited to set and said, "nope. That design is going back to casting."
Vacuum Casting to Avoid Porosity Problems
This is why jewelry is often cast in a vacuum sealed casting machine, which replaces the oxygen in the chamber with argon gas. Metals -- particularly those used in jewelry -- tend to bond very readily with oxygen. Argon, however, is a very non-reactive gas and will not bond with the metal being cast. While some small amount of oxygen may still remain in the chamber, the vacuum sealed machines are quite efficient, and porosity is rarely a problem during the casting process.
Jewelry Repairs - Why Porosity Rears its Ugly Head
Jewelry repairs that are done by hand cannot be done in an oxygen-free atmosphere – that would suffocate your jeweler! Repairs often require pieces of metal to be connected through open flame soldering. The soldering process requires high temperatures, which greatly increase the speed at which oxygen reacts with metals. In other words the hotter the metal gets, the more it pulls in oxygen.
With open flame soldering, it is impossible to only heat the area that is being worked on. Inevitably, surrounding sections of the jewelry will be exposed to high temperatures, causing them to absorb oxygen as they heat and release it as they cool. This is the primary reason for porosity spots on jewelry that has been repaired.
That Ring of Yours Presents a Minefield of Hidden and Unseen Pitfalls for Your Jeweler
Many pieces of jewelry contain elements that are joined together to create a whole piece; a prong setting or a decorative piece might be added after the original jewelry piece has been created. The areas where connections are made during the soldering process are known as "joins."
There are three varieties of solder used to create these joins: soft, medium, and hard. Soft solders use metals with low melting points, while hard solders use metals with high melting points; medium solders, naturally, fall somewhere in the middle.
When repair work is done on jewelry, previous joins can be undone by the heat work of the current repair. This is a source of jeweler nightmares, and care must be taken so that the previous joins are not undone.
Flirting with Porosity - When to Use Soft, Medium, or Hard Solder
Soft or medium solders are typically used in jewelry repair so that the joins of previous work are preserved. When soft solder is heated and then cooled, problems with porosity are nearly inevitable. It’s a bit less of a problem with medium and hard solder.
When re-tipping prongs, jewelers sometimes solder pre-made prong tips to the head or base of a ring in order to create a neater finished product. However, if the entire new prong tip is made up of solder (which is more common than you might think), there will probably be visible porosity throughout the remade tip and often an unevenness in size.
If Soft Solder Causes Porosity, Why Use it?
Soft solder is advantageous in many circumstances because of its relatively low melting point. If your jeweler does not want to conduct heat to sensitive gemstones on your ring, soft solder can allow them to work quickly.
If your piece is particularly delicate and composed of many thin elements, soft solder may be your jeweler's only option to avoid damaging your jewelry.
For re-tipping in particular, soft solder is also a cheaper option that can cut costs on an otherwise expensive repair. While the cheaper option will always be enticing, the lowest price re-tipper may not be the one you want working on your ring.
Can my Porosity Damage be Fixed?
Yes! But it can be expensive. Porosity damage is often done through the use of laser welding. Because of the cost, laser welding may not be a viable option for everyone.
However, if porosity is really uglifying your jewelry or compromising the integrity of joins that hold expensive diamonds in place, laser welding is a great way to get your jewelry fixed.
This All Sounds Scary - Why Write About It?
Full disclosure -- my husband thought that writing this was a terrible idea and that I’d needlessly freak people out about a fairly normal problem that can happen in casting and repairs. Porosity is not a good thing, but neither is it a disaster. As I've tried to explain in this blog, the main "danger" of porosity is simply that it's unattractive.
The key is that, structurally speaking, it's OK in small amounts. While it's not ideal in terms of looks, I've had clients request multiple expensive, time-consuming repairs simply because they feared for the structural integrity of their jewelry. While I'm happy to work on their jewelry, I first need to make sure they realize that they're spending their money on a cosmetic fix rather than a structural one.
Now that I've responded to hundreds of great questions through my blogs and comments, I wanted to address a less frequently asked question. Not many jewelers talk about this stuff. Who even knew it was called porosity? I love to put my clients and readers in the driver’s seat. I want them to know what porosity is, when it's a problem, and what they can do about it. If they care.
I’d love to know if you’re with my husband and think that I should just take this post down, or if you actually find the information helpful. Please talk back in the comment section below. I love your feedback, questions and opinions!
Your Personal Jeweler,