I have recently started doing henna on objects other than people. SUPER FUN!
I'm still very new to this, but I want to share what I've learned so far.
When you henna objects, you have the choice of leaving the henna paste on or scraping the henna away and using the henna stain as your art. just like you do on the skin.
When you want to leave the henna in place such as the Unity Candle and the Canvas I hennaed, I've found my traditional Beachcombers Favorite Henna Recipe works well. If the henna will stay on the item, it's important to not have too much cracking and for the henna to adhere well. The sugar in the Beachcombers Favorite Henna Recipe helps with this.
Henna Unity Candle
I ended up using a clear acrylic based sealer (don't use lacquer sealers as they can be flammable, even when dry). I sprayed five thin coats after the henna was completely dry, letting the sealer dry in between coats.
I think Modge Podge may work beautifully as well, so I may try that next time.
Henna Canvas Painting
Like the candle, I let the henna dry completely and applied a clear acrylic sealer of the entire canvas in light even coats. As I recall I did about 8 thin layers of sealer because I had to ship it from Orlando to my mom in Ohio. It arrived safe and sound with absolutely no henna having fallen off!
UPDATE! March 2014 (3 years later)
This picture is still hanging on my Mom's wall with all henna attached. There isn't a single piece that has fallen off!
Henna Wood Box
This was a small balsa wood box I got for my Grandmother for Christmas and I hennaed it up! I decided that I was going to remove the henna instead of leaving the paste on the box.
I used my normal Beachcombers Favorite henna mix, but the next time I henna something with the intention of removing the henna, I will mix henna without any sugar in it so that it comes off easier. Applying dry heat helped to remove the henna.
I let the henna paste sit on the box for a bout a week then I removed the henna the day before I saw my Grandmother. The stain was a nice warm brown. The next day, I noticed the henna had already darkened slightly. I expect it will continue to darken over time just like a stain on the skin.
UPDATE! January 2012 (1 year later)
This Christmas, my Grandma had the box sitting out for decoration and the color has deepened to be so dark it's nearly black. It's a very deep walnut brown.
So there it is! My experience with hennaing objects so far! I plan do do a ton of other fun things with henna throughout the year, so I'll post more ideas and tips as I learn. I hope I'll hear from those more experienced than I, so that I can avoid mistakes or "learning opportunities" as I like to call them.