More than anything else, anvil bases need to be secure. Naturally, you don't want your anvil to bounce when you really have to wail on it and so ideally, a good base will also have a small amount of flex so that they can absorb some shock.
A wide variety of materials can be used to build them and common examples include tree stumps, stacked lumber, I-Beam and structural steel tube drops. The former two can be found just about anywhere at little or no cost. Call around to tree service companies and ask if you can get a stump the next time they cut a tree down. The latter two can be more of a challenge to find and can get pricey. Ask you local steel supplier if they will sell you their drops, chances are they will for the cost of scrap. They might even just give them to you. Of course, availability is always an issue, so if you really want a steel base, you may have to check in frequently.
I have a small junk yard attached to my shop that is full of stuff left by previous tenants. Needless to say it had been a gold mine to me and my shop mates. I found a 6" x 10" beam buried on a rack that I though would be perfect for this project.
Here is what I did with it:
1. First I cut the beam into two sections of the appropriate length.
2. Next I found a length of 1/4" x 2" steel flat bar, which I cut into 4 equal lengths. These would become the brackets that hold the beams together.
3. I laid out the position of the brackets, drilled and used lag bolts to join the two beams into one solid block.
4. After that, I placed the block onto a 13" sq x 1" thick steel plate.
5. I located the center of the plate and welded two pieces of 2" angle iron to the steel base. These secured the wooden block to the base.
6. Switching gears, I cut four additional pieces of the 1/4" x 2" and welded them into a "T" configuration. These became the mounting brackets for the anvil itself.
7. Once I centered the anvil on top of the block, I lagged the "T" sections into position on the side of the block.
8. I then heated the bars and forged them to conform with the contour of the anvil base. This provided a very solid and secure mounting.
8. Lastly, I sealed the whole thing with a paste wax to protect it from any water that might spill over from my quenching tub.
Whether you are a blacksmith who forges large sections of iron or a jeweler who sets precious stones, a properly secured anvil is of the utmost importance to your productivity.