I want to anneal a piece of coil spring from a 1/2 ton truck to forge into a knife someday. I'm new enough to blacksmithing that I have a hard time judging colors sometimes, even using a temperature color chart.
Would I be right in thinking that if I'm not quite sure of the color by using my judgement that I can test the piece of spring with a magnet and if the magnetizing is gone out of the piece I can start the cooling down process?

Yes, using a magnet would be close enough. The trick to a good anneal (or any heat treat) is to get the whole piece to the same color and soak long enough so the interior is also to temp. If you have a gas forge, this is easier because you can soak it and then turn off the forge, which will accomplish a decent anneal on most steels. With a coal forge, get it to the right temp and stick the piece in old wood ashes or something similar.

What H Wooldrige said. Heat to non magnetic, let is soak at that heat till it is heated clear through then cool slowly in a bucket of ashes, vermiculite or lime work good to, but I just use wood ashes from the fireplace. It should be completely buried in the ashes, let cool overnight then it is ready to forge.

Woody

Also, keep in mind that spring steels often do not like to be overheated and should generally be kept below a bright-yellow heat.

5160 is fairly simple to anneal as has been described. Slower cooling can be achieved by heating a heavy mass of steel along with the spring and putting them both into ashes. The extra heat of the larger mass will slow the cooling rate down in the spring. It really is not necessary to anneal prior to forging. I just cut of a turn of the spring, heat it, straighten it, and start pounding. Maintain a proper heat for the carbon content; high carbon steels do not like to be smacked when they are at a black heat. Normalize once or twice after forging if you plan to heat treat the object. :D

If you have a gasser that you can close up completely with a scrap of Kaowool or some hot brick, you can anneal to a reasonable degree any piece of 5160 that will fit inside. Just put it in, wait for it to get hot (nonmagnetic... just go for a nice lemon yellow heat), seal the opening with the kaowool or brick, and shut off the gas (quickly!). You should probably also cover your burner intakes if you are using a venturi burner setup -- I dunno how well this would work with a blown burner setup, maybe you could fully close the choke or something. Anyway, this is how I do it, and I know that when I do this, the pieces come out drillable soft. :)

I use clean automotive floor dry or kitty litter for my annealing "ashes". It's easy to get, inexpensive, and can be used over and over.

I use Lime / not the pellets but the powdered type, sometimes the piece will be to hot to hold the next day / It is a long slow cooldown that will anneal most anything and it is cheap and reusable over and over

to anneal i use just simple wood ash, never had a problem, matter of fact i annealed a nice sized peace of chaft just the other day :)

Hey woodtick, if you're forging with charcoal, and your forge is large enough to hold the whole piece, after you've soaked at temperature, just turn off the blower and walk away. Make sure it's good and buried in the coals. Leave it overnight, that'll be good. When you dig out the steel the next day, it'll appear to be cool, but it'll still burn your hands.

I use vermiculite. Today I went buy another bag and was told by the sales person that they pulled the vermiculite because it was found to be carcinogenic. Has anyone else heard this or was the sales person full of ----? What is the best alternative to vermiculite?

I would only anneal if you planned filing the whole thing. I dont think it required before forging, at least I don't

What is the best alternative to vermiculite?

Wood ash is probally the cheapest way to go

If I remember correctly a demonstrator about 20 years ago recommended saving the scale that came off onto the anvil while forging. He used a bucket full of scale for annealing.

I use vermiculite. Today I went buy another bag and was told by the sales person that they pulled the vermiculite because it was found to be carcinogenic. Has anyone else heard this or was the sales person full of ----? What is the best alternative to vermiculite?

I checked today at home depot. They still sell vermiculite in the garden center. Maybe it is a local thing?

I checked the home depot. They still sell it in the garden center.
It is still used as a packing material.
Contact Air Sea Containers

Good info, got some coil springs, also wanting to make something out of them

Some vermiculite contains trace amounts of asbestos as does the soil of any place around a roadway that is old enough to have had asbestos brake pads used by cars traveling on it.

What about Pearlite? (they mine it about 2 miles from where I'm sitting...)