Shrinkage - In Depth

Shrinkage Square jpg

Personally, I feel shrinkage is the most challenging variable when working with metal clay. It impacts not only size but also construction, embedded or attached elements, and design. Shrinkage alone is probably worth a book, so I am covering only the major aspects of it. And, I am figuring it out as I write. The subtitle of this post should be “More Questions Than Answers."

The basic phenomenon of shrinkage occurs when the silver sinters, thereby removing water, binder, and space between the molecules of silver. The silver molecules then get packed together tightly and share electrons and bonds, which is what makes it hard and shiny. Silver metal clay shrinks toward the area of highest mass. If you had a ball of silver, it would shrink evenly toward the center. If you had a donut of silver, the donut ring would shrink into itself, but the hole would not shrink as much and certainly not proportionately to the donut part. This means that holes and thick or thin parts in your creations do not follow the simple math found in a statement such as “PMC3 shrinks 12%.” And this is why manufacturers will often give a range for shrinkage, such as, “PMC3 shrinks 12 to 15%.”

An unrecognized variable of shrinkage is the clay's hydration and composition. This means PMC3 fresh out of the package might shrink 12 to 15%. Say I take that PMC3 and roll it numerous times on a texture that I’ve treated with olive oil. Then, since it feels a little dry, I add some water and knead it. This PMC3, which now has more water and oil than the fresh clay, might be larger in the wet clay form. It might therefore mathematically shrink more to its fired form when the silver is compressed.

For me, there are two ways to look at shrinkage during the construction steps. Shrinkage and enlargement. 

Shrinkage is the difference between your wet-clay piece and the final size of your fired creation, when it is finished and polished and admired, worn, or used. Shrinkage is most often expressed as a percentage. Divide the percentage by 100 to get the decimal form.

Shrinkage is calculated as follows.

  1. Fired piece measurement minus wet-clay piece measurement = Change-in-size (a negative number)
  2. Change-in-size divided by the wet-clay piece size
  3. Times this by 100 to make it a percentage.

Enlargement is the factor you use to calculate how much larger your wet-clay piece needs to be to result in your desired fired-piece size.

Enlargement is calculated as follows.

  1. One (1) minus the manufacturer’s shrink rate in decimal form = Decimal
  2. One (1) divided by this Decimal result = Enlargement factor (always larger than 1)
  3. Desired fired piece size times Enlargement factor = the wet-clay piece size required

Tiny Example

Here is a simple example. Here is a little charm made with gold metal clay and fine-silver metal clay. The gold reported shrinkage is 12%; the silver 12 to 15%. My wet-clay charm measured 12.9 mm, as best as I could determine. The dry-clay charm measured 12.8 mm, as shown below.

Shrinkage pre 12.5 mm

The fired charm measured 10.2 mm, as shown below. 

Shrinkage post 10.0 mm

Shrinkage is calculated as follows. 

  1. Fired piece measurement minus wet-clay piece measurement = Change-in-size
  2. Change-in-size divided by the wet-clay piece size
  3. Times this by 100 to make it a percentage.

Shrinkage — Charm Example

  1. 10.2 - 12.9 = -2.7
  2. 2.7 / 12.9 = .209
  3. .209 X 100 = 20.9%

My charm shrank 20.9%, way more than the manufacturer’s suggested 12 to 15%.

Enlargement is calculated as follows.

  1. One (1) minus the manufacturer’s shrink rate in decimal form = Decimal
  2. One (1) divided by this Decimal result = Enlargement factor (always larger than 1)
  3. Desired fired piece size times Enlargement factor = the wet-clay piece size required

Enlargement — Charm Example

  1. 1 - .12 (12% shrink rate) = .88
  2. 1 / .88 = 1.14
  3. If I wanted to make a fired piece that measured 10.2 mm, I would have times it by 1.14. This would have left me making a wet-clay piece that measured 11.6 mm (10.2 X 1.14 = 11.6 mm).

But my charm’s actual enlargement factor calculated to be 1.26, not 1.14.

Bottom line is my charm shrank way more than expected.

Why did my little charm shrink more than the manufacturer’s shrink rate of 12 to 15%? It’s a very small piece. Remember I mentioned the clay's hydration and composition as a variable? Fresh clay might shrink according to plan, but used clay — dryer or wetter or with compromised binder or additional anti-stick — may not. 

In this charm, I used fresh PMC3 but I had added water to the gold, as the package insert suggested. Because I was experimenting with the ability of gold clay to adhere to silver clay, I let water sit on this piece sit for a long time, like 15 to 30 minutes. (By the way, despite the gold’s package insert claim and my attempt the two metals did not stick to each other.) Lastly, I had added PMC Syringe on the front and the back and let this sit saturated with water for many more minutes. But I dried the charm, right? Yes. Maybe it does not shrink when simply drying as much as a clay that starts out dryer. And my measurements began with wet-clay measurements.

Conclusion Shrinkage sucks. It is unpredictable and unreliable. At best it might be predicted by math and at worst it defies all calculations and wastes a lot of resources — silver and gold. 

This explains to me why my gemstone settings are most often on the small size. My clay must shrink more than usual. Maybe it’s my elevation. Maybe it’s how I hold my tongue. If you have any comments or information that would be helpful for others and me, do share!

Large Examples

I happen to be making two pendants to replace some pieces that sold — one Condor and one Eagle. I made them out of “960” clay, which was made by mixing 55.5 grams PMC3 (99.9% silver) with 57.0 grams PMC Sterling (92.5% silver). PMC3 shrink rate is 12 to 15%. Sterling shrink rate is 15 to 20%. I did the math here and the supposed shrink rate for my “960” should be 15.3%.

— Eagle Pendant

This pendant is constructed so that the largest mass of clay is toward the center of the square part. (The smaller numbers in the graphics are the side thicknesses, but these were too small to measure and use for accurate calculations.)

Here are the side measurements in the dry-clay (not wet clay) stage.
Top 32.0 mm; R Side 33.0 mm; Bottom 33.0 mm; L Side 32.5 mm

S Eagle pre

Here are the measurements in the fired or finished stage.
Top 27.0 mm; R Side 27.4 mm; Bottom 27.0 mm; L Side 27.0 mm

S Eagle post

The numbers in red below show the shrink rates for each side.

S Eagle Shrink Rate jpg

My calculated and expected shrink rate was 15.3%.
In this piece, my average shrink rate (of the four sides) was 16.9%. Again, more!
The top shrink rate of 15.6% was smaller most likely because the addition of the bail, which physically restricted shrinkage underneath it.
The sides shrank the same, which makes sense because their thickness and composition were roughly the same.
The bottom shrank most off all. This area had more clay mass than the sides. Is that why? More mass means more shrinkage? I thought more mass was a directional thing. I shall point out here that Eagle was wider at the bottom in the dry-clay stage and then equal top-and-bottom after it was fired.

— Condor Pendant

This pendant is constructed so that the largest mass of clay is toward the lower right corner of the square part.

Instead of measuring, I compared the shapes visually. My theory is the pendant will shrink lopsidedly, but I have no idea how.

Here is the measurement in the dry-clay stage.
Top 35.0 mm

S Condor pre

Here is the measurement in the fired or finished clay stage.
Top 29.9 mm

S Condor post

The number in red shows the shrink rate for the top.

S Condor Shrink Rate jpg

The actual shrink rate of 14.6% is smaller than my calculated shrink rate of 15.3%. Less? Argh! Such inconsistency. This bail was not as tall as Eagle's but a couple mm’s wider. Could this have restricted shrinkage more? 

This Condor pendant does contain 5.21 grams more clay than the Eagle. Does more mass mean it should shrink more?

I made these two pendants side by side, so one could rule out other conditions that may have contributed to this inconsistency.

I wish, oh I wish I had measured left, right and bottom sides because my guess is the lower right corner would have had the highest shrink rate, because it is quite thick and the area of highest mass. In lieu of measurements, I will offer this graphic, which includes a photo of rust-colored fired Condor overlaid upon a photo of the dry-clay Condor.

Shrinkage overlay Condor jpg

Visually, you can decide yourself if you think it shrank proportionately and evenly. 

Again, I can only conclude with regards to shrinkage that it might be best to fly by the seat of your pants. Know that shrinkage depends on many, many variables, and most of all these.

  • Construction, as in thick and thin
  • Construction, as in embedded or attached elements
  • Construction, as in holes shrink less than solid parts
  • Clay, as in fresh or wet, dry, or with additional anti-stick

One More Example

Here is another pendant with shrink rates. Again, my shrink rate is higher then the expected shrink rate for PMC3, which is what it is made of (along with PMC Paper).

Dragon Shrink Rates jpg smaller

Shrinkage and Enlargement Chart

For what it’s worth, here is a shrinkage and enlargement factor chart for the more common silver metal clays (and gold clay).

Shrinkage and Expansion Charts jpg

And this concludes everything I know about shrinkage. For myself, I think I raised more questions than provided answers. If any of what I shared here helps you in any way, for that alone I am happy.

For online courses in metal clay, go to I Love Silver, where you learn how to design and create your own silver creations.

© Kris A Kramer 2019