Over at the Casa de Solariego blog, which is chronicling the building of a straw bale home in San Luis Obispo County, blogger Mark explains how he stained his new concrete floor with ferrous sulfate, a common fertilizer, for only five bucks:
"We started out by mixing up ferrous sulfate and diluting it in water. We then strained it into a garden sprayer and just sprayed it on the slab. The ferrous sulfate (iron really) reacts with chemicals in the concrete, and turns it a multitude of colors from yellow to orange to red to brown. We got all kinds of veining and the cracks turned dark and got accentuated, so it looks like marble now.
"The pictures don’t do it justice, it’s very beautiful. Also, we did the great room a little darker (extra coats) and haven’t sealed that area yet (the sealing makes it darker still). Should look very nice.
"The stuff is very cheap. I figure it cost about $5 to do both units. Of course, the acrylic sealer was $87 for five gallons. We computed 15 gallons, but looks like we’ll use far less."
I’m not sure how you would do this to an existing floor with the walls already in place. How would you protect the walls? And for sure, you would need super-duper ventilation. Be safe!