On the beaches at Slope Point in the Catlins, Southland, New Zealand, petrified wood of various colours can be found. Perhaps most common is the black variety. Some of the black petrified wood shows some lighter-coloured grain within it but some does not. When wet, the black petrified wood often shows little or no grain – when dry, it becomes a little lighter in colour and may reveal some grain.
Jocelyn Thornton, in her booklet “Gemstones”, has a page on “Beach Pebbles – Slope Point” (see photo below, left, paragraph labelled 1). In there, she states that collectors often place black petrified punga and other stones in household bleach overnight “after which the grain of wood…is revealed” (see page 35 of “Gemstones”). In the Birdlings Flat Gemstone and Fossil Museum, Vince Burke includes an example of Slope Point petrified wood “bleached to bring out the grain” (see photo below, middle). Having heard of this practice mentioned by a couple of other collectors, including Tracey Kidd (photo below, right), I decided to try it out.
In a recent discussion in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”, a member mentioned that he had soaked a piece of black petrified wood “in straight bleach for a few weeks”. In a 2013 discussion in The Fossil Forum on the use of bleach, a UK contributor stated: “If the starting material is black or very dark in colour, it should bring up the grain but you’ll probably need to leave it soaking for about a month (in a non-metal container) and top up with fresh bleach every few days”. In the US-based Rock Tumbling Hobby Forum, one contributor from Oregon showed before and after photos of petrified sycamore which had been polished then soaked in bleach “for a few days” (see photos below, left and middle). On the same Forum, another showed the results of bleaching polished black petrified wood from Montana for three to four days (see photo below right). What emerges from the discussions on these forums is, in general, it seems that people can use bleach either before or after polishing, they often use full strength bleach, it affects only the outer layer of the stone, it is successful for only some types of petrified wood, and the same effect can be achieved by leaving the petrified wood in direct sunlight for a year or more.
I had not read these before I started my own experiment. However, Tracey Kidd had told me she had used a 50/50 dilution of bleach and had soaked her un-polished petrified wood in it for about 18 hours. I selected 12 pieces of black Slope Point petrified wood (unpolished) and decided to try them in a solution of 1/3 bleach and 2/3 water for two days.
The results were mixed, probably dependent on the nature of the petrified wood, how “open” the grain is. This is demonstrated by the largest piece, #1 (11 cms long). [Note: Taking photos of un-polished black petrified wood poses a number of challenges, with the angle to the light giving different results – many of my pieces below looked blacker before bleaching than represented in the photos.] On one side, the smoother side with less grain being revealed, has retained more black than the other side. This is the side that ended up blacker, with one line of grain along it:
This is the side that ended up lighter, with more grain apparent on it:
One side of #9 (side a) remained the blackest after the bleach:
One side of piece #6 showed some brown patches after bleaching:
Piece #12 is something like fossil tree fern and I hoped the bleach would bring out its patterns more clearly. There was a small improvement:
Three of the other stones, #7, #8 and #11:
In some cases, the bleaching these stones has resulted in a grey surface that is less interesting than the original black. In other cases, wood grain patterns have been made clearer and brown patches revealed. Sometime in the future, I will experiment with polished petrified wood and see what happens.
TumbleStone Posts with examples of petrified wood stones from Slope Point: “Twenty-Four Slope Point Stones Polished for Oliver: Part 1, Stones 1 to 10”, “Twenty-Four Slope Point Stones Polished for Oliver: Part 2, Stones 11 to 24”, “Another South Island Fossicking Trip, February/ March 2021 – Days 15 and 16 (Slope Point, Gemstone Beach)” and “January 2022, Stone of the Day #10 – Light-Coloured Petrified Wood from Slope Point”.
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