Iron Sulfate is the cheapest and probably also the least toxic concrete stain ingredient.
You don't even need any acid for this stain.
Iron sulfate is also known as "Copperas" and is sold for the purpose of helping yellowed plants regain a dark green color.
A four-pound bag of "Hi-Yield" brand Copperas costs about 5 dollars.
I purchased one of the bags below at Ace Hardware and the other at a plant nursery.
Mix the iron sulfate into the water in a non-metal bowl. The light yellow powder will dissolve yielding a tan colored solution. Do not mix the solution far in advance of when you want to use it because, over time, rust particles will precipitate out of solution.
The solution was brushed onto 10 square feet of clean gray concrete.
The iron sulfate solution gradually turns rust colored as it is exposed to air. It dries to a nice orange color. In the image below you can see a vertical mark left by a measuring tape that was sitting in the wet stain as it dried.
OK, that looks nice but not all of it soaked in.
There is a fair amount of excess rust sitting on the surface of the concrete.
Attempting to seal over the top of this seems like a bad idea.
The sealant probably wouldn't stick very well.
I wiped up the excess with plain tap water and a bright yellow colored sponge.
As you can see below, there was enough un-absorbed rust to tint the sponge and water bright orange.
Since there was no acid in this recipe, I figured there was no reason to use baking soda when cleaning it up.
This is what it looks like after the excess was removed.
The darker part is wet.
Not too shabby ehh?
The wet look is approximately what it would look like if wet-look sealant were applied.
The vertical mark left by the measuring tape is still very noticeable.
That was accidental, but I have read about people using sawdust, cornflakes, pine needles, and plastic wrap to purposely create interesting marks.
I mixed the same amount of water with only half as much iron sulfate (2 tablespoons) and brushed it out over 10 square feet of gray concrete.
In the mixing bowl, the solution looked almost the same as the first one above.
On the floor however it looked almost like water.
It began to turn orange after a few minutes but the final result after cleaning up was less orange than above:
I am tempted to try halving it again test 1 tablespoon per 10 square feet.
The double-strength recipe contained 1/2 cup iron sulfate with 1 and 1/3 cups warm water spread over 10 square feet of gray concrete.
The double water over the same surface area seemed to be a problem because there was a lot of it that did not soak in by the time the rust color appeared.
Most of the extra iron in this double dose was left sitting on the surface.
The final color after cleaning up the considerable excess was only slightly more intense than the first formula.
Perhaps a more effective means of delivering a "double" would be one single dose allowed to fully dry and then a second single dose.
Iron chloride is also known as Ferric Chloride and solutions containing it are sold at electronics supply stores like Radio Shack -- not consumer electronics stores like BestBuy.
It is sold for the purpose of etching circuits onto printed circuit boards (PCBs).
Don't let them talk you into the new Ammonium Persulfate etchant, that won't stain your floors.
I decided to try a concentration of 1/2 cup brushed over 10 square feet of concrete.
It took about 15 seconds for the freshly spread solution to change color from mustard yellow to rusty orange.
The fizzing was easy to hear so it seems the etching solution was quite acidic.
It also had an unpleasant odor.
It fizzed so much there were bubbles.
The yellow swirls in the image below are made of frothy bubbles.
After it dried, I started using a sponge to wipe up the unabsorbed excess iron.
It was a lot more slimy than the excess that remained on the surface with the other stains.
At first I was impressed by how much deeper the color appeared than with the Iron Sulfate experiments described above.
In some parts of the test area it seemed like the more I wiped the floor, the more "stain" was washed away.
It was as if the top surface of the concrete was dissolved away by the acid, revealing unstained concrete below.
Other portions of the test area had limited surface erosion and showed a great deal of color.
The image below is a close-up of an area with significant surface erosion.
None of these tiny pebbles were visible before the etching fluid was applied to the floor.
They were uncovered by the fizzy reaction between the fluid and the concrete.
I was thrilled with the depth of color imparted by the etching solution but very unhappy with the surface erosion.
This is how it looks wet:
I hope it is possible to find a balance that yields the rich color without the damage. Perhaps diluting the etching solution with water, or simply spreading a smaller amount out over a larger area. I will definitely be doing some more experiments with etching solution.