This was written in response to an email I received that was a little more vehement than most about anvils not wearing out.
"You need to do a little more research on anvils before spreading your bs on the net.
Granted a "small anvil" will not have the mass for larger forgings but "wearing out" an anvil only comes with working iron cold.
A skilled smith doesn't need a mirror faced anvil for forging hot iron."
Anvils like any object made of metal are subject "cyclic fatigue".
In other words the metal wears out!
Cyclic fatigue occurs when and metal object is flexed (deflected) over and over again.
Generally objects like springs and bearings have there life measured in millions of cycles.
Even the surface of a plain bearing that is kept perfectly clean and perfectly lubricated wears out due to the load changing direction with every rotation, microscopically deflecting the bearing surface millions of times.
The first indication of cyclic fatigue is a fine pattern of small cracks.
Later (in bearings) small flakes of metal start to come loose from the bearing.
Eventually these flakes amount to significant wear and the bearing fails.
In high performance applications (high strength alloys loaded to the extreme) the cracks propagate and you have a catastrophic failure (broken shaft).
Springs are also subject to cyclic fatigue.
The interesting thing about steel as a spring material is that ALL steels have the same springyness.
This is defined as "E" the "modulus of elasticity" or "Young's modulus of elasticity" to be more specific.
Young's work being based on "Hooke's law".
For all steel E = 29.8 million psi and is generally rounded to 30 million (the slide rule days)..
This is used to define how much a piece of steel will deflect under a certain load up to the "yield" point.
All steel deflects the same amount under equal loads, the difference being that heat treated high carbon steels have a higher yield point therefore they can be deflected FURTHER before they yield.
Yep, you can make mild steel springs and they work great (My father taught me this but I had to test test it to believe it, Thanks Dad).
The problem with high strength steels is that this ability to withstand high deflection also creates a situation where the part is stressed close to the ultimate limit.
Good springs are designed to take millions of cycles.
As a rule of thumb a part that can take a million cycles is good for infinite cycles.
This rule fails in practice when people (like me) insist on getting half a million miles out of there automobiles and parts see billions and trillions of cycles.
Public works like bridges have the same problem.
Those steel beams and cable will not last forever.
Back to anvils.
An anvil is a specially designed work surface that is designed to act like a spring.
It deflects and springs back returning the energy it put into it.
In blacksmith's jargon a "live" anvil vs. a "dead" anvil.
A dead anvil is soft or non-resiliant.
It absorbs the energy and doesn't spring back.
This is very hard on the smith whom has to lift the hammer off the work every time rather than having it spring back a high percentage of the way.
An anvil with a lot of "life" is very springy and gives back a high proportion of the energy put into it.
This comes at a cost though.
The live anvil is very hard and often subject to chipping.
AND like the springs and bearings described above anvils are subject to cyclic fatigue (YES, they wear out).
My hundred pound KOHLSWA anvil was hard as glass, flat, a thing of beauty.
The only chips on the corners came from the shop out of which I purchased it
(see On Becoming a Blacksmith).
These were the result of an inexperienced striker with a sledge missing the work and striking the edge several times.
The KOHLSWA is a Swedish cast steel anvil and herein lies some of the problem.
When I sold my KOHLSWA I had started to use a slightly heavier M & H Armitage "mouse hole" anvil.
I would not have sold the KOHLSWA except that it had started to show a fine pattern of little cracks all across the center of the face.
I recognized the cyclic fatigue cracking and was both shocked and disappointed at the time.
But then I thought about the class of work I was doing and the face that I had worn out two hammers and was working on a third.
It made sense that I had worn out the anvil too.
This brings up the difference between cast steel anvils (the only type available NEW today except Peddinghaus) and those with a forge welded steel face on a steel or wrought iron body (Hey-Buddens, mouse hole, ect.).
When the cast steel anvil starts to show cracking those cracks will slowly propagate further into the anvil.
Eventually a big chunk is going come off.
When the two piece top starts to crack it can only propagate until it hits the soft wrought body.
Over the years I've had and used dozens of anvils in my shop, and used more in others shops.
As a life long student of early technology I study every tool I can quite closely.
I am a keen observer and understand the mechanisms of failure.
All tools eventually wear out.
Anvils wear out.
Anvils wear out faster when too small an anvil is used for a given class of work.
A 100 pound anvil was too small for the class of work I was doing.
Today I have a 325 pound KOHSLWA that will last my lifetime and probably several others.
If you would like to know more about the strength of materials the classic reference is:
Elements of Strength of Materials,
Timoshenko and MacCullough,
D.VanNostrand Company, Inc., 1935.
OBTW, Robert. A rough surface on your anvil will be imprinted on every surface that is forged on it.
NOTE: This is not the Robert Bastow who asks the gas forge questions and will soon be capable of telling me a thing or two about gas forges.