G.A., Windham, N.H.

A. It should be safe because muriatic acid attacks lime (and will help dissolve that hardened mortar), and there is no lime in the plastic. The acid could etch any metal, say, where the plastic connects to copper or cast iron. Just be careful pouring the acid into the drain; keep it off the metal rim around the drain opening. And thinking about acid, it occurs to me that nonenzyme Drano, which is an acid, has been used for years without damage to pipes of any sort.

Dilute the acid half and half with water. Always pour acid into water. Pour the acid solution very carefully into the drain, making sure it does not splash. A funnel will accomplish this. Wear skin and eye protection when working with acid.

Grave error Miguel Mitchell of Rockport wrote to tell us of a grave error the handyman made in suggesting ways to clear a drain of mortar that was spilled into the drain. This question was asked by G.A. of Windham, N.H. in the Nov. 8 zonal Home & Garden section.

The handyman suggested using muriatic acid, which would attack the mortar without harming the drain. He also said that nonenzyeme Drano might also work.

Dr. Mitchell, visiting scientist and chemistry lecturer at Northeastern University, wrote that the handyman described ``nonenzyme Drano'' as an acid. ``Actually, the active ingredient in Drano is sodium hydroxide, a very powerful base,'' he said.

``If someone decided to combine Drano with muriatic acid to clean pipes, they would be in grave danger. Strong acids and bases react violently. In combination with muriatic acid, Drano would erupt into a spray of superheated, corrosive steam. Let your readers know that Drano is not an acid and should never be combined with muriatic acid.''

Excellent point, Dr. Mitchell, and appreciated by the handyman. At least the handyman did not suggest combining the two materials. In fact, one should never combine two materials designed to do the same thing, or for that matter, any two cleaning materials.

Q. I have heard that muriatic acid will remove cement that got brushed onto a patio when the bricks were remortared. Does it work? How is it used? And how long does it take to remove the cement?

R. MURRAY CAMPBELL, Cohasset

A. You heard right, but your other questions are interesting because there are no directions on the acid bottle as to how to do the treatment. Muriatic acid attacks the lime in cement and mortar, and that is how it removes it. How to do it? Mix the acid 1 to 1 with water (pour acid into water; doing it the other way will cause splashes, not a nice thing with acid. And, as with all acids, wear skin and eye protection when working with them.) Brush on the solution with a paintbrush or pour it on the smeared cement or mortar. It will fizz up for a few seconds; when the fizzing stops, rinse. Scrubbing with a scrub brush or wire brush will help remove stubborn spots. Repeat as necessary. Let the rinsed area dry off completely before repeating.

Incidentally, you can do all this the day after the work was done, or 10 years later; time is no factor.