What is the strongest edible adhesive that will work on gingerbread? I'm thinking this Christmas I might use it for making a gingerbread house. And how cohesive is gingerbread, because it might break easily?

-- Nathan, age 12

Don't worry, Nathan. Gingerbread houses rarely have to withstand earthquakes, so a moderately strong adhesive will do.

Of course, the odds are pretty slim that a gingerbread house will actually be eaten -- by humans, that is. (If you keep your masterpiece from year to year, store it in mouse-proof packaging.) So when it comes right down to it, your glue doesn't have to be edible. Nevertheless, it's simply good practice when making something out of food to use only edible components, so your edibility concern is quite appropriate.

The best cement for such a construction project is known as royal icing. It's not only edible, but it also dries rock-hard. Here's how to make it: In a medium bowl, mix 21/3 cups confectioners' sugar,

1/2 teaspoon cream of tartar, 2 egg whites (their albumin proteins are the actual glue) and 3/4 teaspoon vanilla extract, only because we both know you're going to lick the bowl. Beat the mixture with an electric hand mixer until it is thoroughly combined. If it's not a nice, thick paste, thicken it with a bit more sugar or thin it with water a drop at a time. You can color it with food coloring, if you wish. It will keep for several days in the bowl (no need to refrigerate it) if covered with a damp cloth.

Regarding the gingerbread panels themselves, just be sure to make them from a relatively dense dough, rolled out to a uniform thickness. Your house will be sturdy enough to withstand the Big Bad Wolf's most powerful huffs and puffs.

In baking bread, is it safe to use unglazed stone or clay tiles, such as one would find at any Home Depot, in place of the baking stones sold in kitchen stores? Could such tiles allow harmful elements or compounds to leach into the food that is baked atop it? The cost differential favors -- in some cases, greatly favors -- the tile, but I don't want my frugality to come at the expense of food safety.

Unglazed quarry tiles, the dark red, 6-by-6-inch flooring squares sold at home improvement stores for about $1.20 per square foot, are made by extruding natural clays or crushed shale into half-inch sheets and firing them at temperatures as high as 2,000 degrees. Because they are made from natural materials, their chemical compositions will vary, including the possibility that compounds of toxic metals such as arsenic or lead might be present. That's of no concern to the flooring manufacturers, but the tiles might not be entirely safe for contact with food.

The word "unglazed" is critical, by the way, because glazes -- smooth glassy coatings, especially the yellow ones -- often contain lead.

There's no doubt that quarry tiles will work well to help bake breads and pizzas develop a crisp crust. During my unsuccessful efforts to produce a good, French sourdough baguette, my oven rack was covered with them. Although I am obviously still alive, I cannot draw any general conclusions about the safety of the tiles. However, lead poisoning is cumulative, and one of its major symptoms is a lowering of IQ. Have my columns been getting more and more dimwitted?

When I received this question, I ran out to Home Depot, bought one quarry tile and checked its surface with a sensitive test for lead. The test was negative; no lead.

Nevertheless, my advice is to spring for one of those commercial baking stones at $25 and up. They're undoubtedly made from nontoxic materials.

Thanks for the bleach cleaning tips [Aug. 31]. However, I have this vague memory of being warned not to mix chlorine bleach with ammonia -- that when you combine them you come up with something dangerous that can knock you out -- or worse. Shouldn't you warn your readers not to mix bleach with just any old household chemical?

I did advise my readers to observe the cautions on the label of the bleach bottle. They say, "Do not use or mix with other household chemicals such as toilet bowl cleaners, rust removers, acids or products containing ammonia. To do so will release hazardous, irritating gases."

That's a pretty vague warning, which is why your memory is equally vague. How is a consumer without a degree in chemistry to know which "household chemicals" are dangerous and which are not?

Here's the scoop.

Chlorine bleach, a solution of sodium hypochlorite in water, will release chlorine gas at the drop of a hat -- or an acid. So toilet cleaners, which often contain hydrochloric acid, will release this highly irritating gas that can cause eye, nose and throat irritation, breathing problems and, eventually, pneumonia. (Chlorine was used as a poison gas by the Germans in World War I.) Rust removers are likely to contain oxalic acid or phosphoric acid, so mixing chlorine bleach -- or any drain cleaner that contains sodium hypochlorite (read the label) -- with any of these acidic products is risky. If a chlorine smell begins getting to you, go outside and breathe fresh air as soon as possible.

Ammonia is even worse than acids. Chlorine reacts with ammonia to produce chloramines, gases that induce coughing, nausea, shortness of breath, and, in very large doses, death.

So look for ammonia on the labels of cleaning products and keep them away from contact with chlorine bleach.

Again, read the labels. Or if you're severely chemically challenged, just don't mix chlorine bleach with anything, with the exception of a detergent in your washing machine.

LABELINGO: Perspicacious reader Mary Brown of Springfield bought a package of Esprit de France Petit Pain Partially Baked French Style Rolls. She says the rolls were okay, but not great. Then she read the directions more carefully: "Remove the bag, and bake rolls in a hot oven (400 C) for 12 minutes or until crust is golden brown."

"Gee," she writes, "if only my oven reached 752 degrees Fahrenheit, the rolls might really have been great."

But black, instead of golden brown.

Robert L. Wolke (www.robertwolke.com) taught chemistry at the University of Pittsburgh. His latest book is "What Einstein Told His Cook 2, the Sequel: Further Adventures in Kitchen Science." He can be reached at wolke@pitt.edu.