Goldenrod Paper Indicator with a Black Twist

Goldenrod-colored paper once came with a pigment formulation that was pH sensitive.  Apparently similar to pigments present in turmeric, these paper pigments were yellow in an acidic environment (like vinegar) and red in a basic environment (like ammonia).  This goldenrod paper was long used as big indicator strips for pH demonstrations, including hidden messages written in wax and developed with ammonia solution and "bloody" handprints made from ammonia solutions.  See http://www.stevespanglerscience.com/lab/experiments/goldenrod-paper-message and http://blog.teachersource.com/2014/10/23/seeking-goldenrod-paper/ for more details. Here is an example of a wax-written message.

The formulation of the paper has changed (apparently the old formulation was not very recyclable) and the dyes are no longer pH sensitive.  Many science demonstration suppliers stockpiled the paper with the old formulation for a time, and there have been some shortages in the past.  There is a way to make a sort of goldenrod paper using turmeric itself, see http://blog.teachersource.com/2014/02/05/make-goldenrod-paper/.   Here is an example of turmeric (an old sample from the collection of the late great Doris Kolb) in various solutions: yellow in vinegar, reddish in ammonia, and dark brown in 6 M hydrochloric acid.

I still possess many sheets of the old goldenrod paper, which I use for demos and will now guard more carefully.  So, what is the black twist?  A couple of days ago, I accidentally put a piece of old goldenrod paper in contact with 6 M hydrochloric acid solution and it turned black!  Unlike with ammonia solution, the color did not readily fade when the solution dried.  Areas of the paper adjacent to the blackened locations also darkened, possibly due to diffusion of HCl vapor. The yellow color can be largely restored by spraying with vinegar or with ammonia solution (it turns red and then fades back to yellow).  I have not been a fan of the "bloody handprint" demo, at least the "bloody" analogy, so we have often attached pipe cleaner "legs" to turn a handprint into a hand turkey.  The ability to make the paper black will now allow me to use hydrochloric acid to draw the legs, an extension of the indicator theme. Here is a test run: first, an ammonia handprint with some hydrochloric acid highlights; second, the ammonia leaves after a time and the black features become more diffuse; and third, after spraying the left leg with vinegar and the right leg with ammonia solution and waiting, colors have somewhat returned to yellow.