Many people have posted here about issues with their royal icing, especially black and dark colors. There are always many helpful suggestions, but I don't see many follow up posts to let us know what worked, and I am curious. Have you had a problem that remains a mystery, or one you solved by making a change, and what fixed it?
There are regular questions about r.i. crumbling, breaking, falling off, or simply having a strange or rough texture, becoming crystallized, more bubbles, being matte or shiny, or even not drying, and taste, among other issues with it. Suggestions of the cause are all over the place, and sometimes even polar opposites. I am starting to think some of what I do is nothing more than cookier superstition, lol! Here are just a few of the suspects, and please jump in if you have any difinitive experience with any of them, even if you only know the cause but not the solution.
Brand or old age of meringue powder, using powdered, fresh or pasturized egg whites, adding or omitting corn syrup, glycerine, cream of tartar, flavored non-dairy creamer, white food coloring, black food coloring, the type (such as gel, paste, powder, etc.) or brand of coloring, extracts/emulsions or lemon juice, change in ingredients or ratios of recipe, or even when using glaze or a mix of glaze and royal icing, etc. (We all know oily is bad, but why exactly?)
Under-mixing, over-mixing, handheld electric mixers, beaters, whisk or paddle attachments, thickening with powdered sugar, thickening by whipping for a long time, thickening and then thinning back out, re-use of old icing, length of storage, refrigeration, freezing, mixing separated icing, heat from hands, environmental humidity or dry air, temperature, air movement such as a fan, dehydrators, drying time, using fresh from the oven cookies, etc.
What works for you? What caused a disaster? What advice do you ignore, yet everything works great? What issues are still elusive, without a known cause or solution? Are there other ingredients, procedures, or environmental factors you suspect are a problem? Altitude? Witchcraft?
Please take a moment to weigh in, and I might just do some experiments to try and solve anything we are still perplexed by. 
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