Which burns more?


SGF, Supreme Grumble Framer
Nov 19, 2003
Orange County, CA
You might figure that you'd get more acid burn from wood than paper ... but not in this example!

Here's how the corners were finished (plywood).

And here's another photo after lifting off the plywood ... you can see that there has been less damage under the plywood than from the paper dustcover.

Perhaps a compelling argument for using Lineco AF paper for dustcovers!
I can't buy that one, John, the whole thing was covered in black kraft paper. There shouldn't have been very much light getting to either the foamcore or the wood corners.

Maybe that proves that black kraft paper is really BAD!!!

Was the plywood corner any lighter color on the inside??? Perhaps the black kraft paper darkened the plywood also!
Plywood same color both sides ... not colored / darkened from black kraft.
Uh, would you believe, .............. Archival plywood???


Maybe the used the cheapest wood they could find to make the plywood (really BASE material!)

OK was the plywood polyurethaned? You get more burn at the exposed edges than along the grain. I know the knots and cracks on old frame backing is the site of more damage when I open the really old frames at least.

I tend to thnk it is more oxidation and the lack thereof under the plywood than acid burn that has discolored the backing.

Hope this thread grows as I look forward to the varied opinions that will pop up!
It only looks nicer on your white walls.. :D

The "Plywood" corner that you're showing Andrew is Mahogany, which is neutral. Unless you're on the FACTS board, then its naughty naughty naughty.

As you have found out from REAL life experience... not only is the mahogany neutral but also the ATG adhered very nicely to the wood.

And I'll end my little diatribe here. :D
The "Plywood" corner that you're showing Andrew is Mahogany, which is neutral. Unless you're on the FACTS board, then its naughty naughty naughty.
Baer ...

I'm afraid I don't understand what you are saying here:
(i) maybe the plywood is made from mahogany but it is definitely plywood (3 ply to be precise)
(ii) The average natural pH level of wood is between 3 and 5.5pH. Why is mahogany different / neutral?
(iii) Don't understand the 'FACTS board' comment at all ... maybe I'm slow this morning ... haven't had my coffee yet!

Please extend the diatribe a little more. Many thanks as always.
Here's a wild guess. The browning from acid migration is an oxidation process (right?). The corner brace served to restrict the airflow to the backing it covered so less oxygen was present to combine with the acidic gasses and oxidize the backing.
When taking apart a vintage print that has spruce panel backing you notice that the most highly oxidized areas are those that correspond to the gaps between the panels.
What I think you are seeing is the corner bracing material providing a physical barrier thus restricting the chemical reaction. An interesting study would be to make a similar frame and brace the 4 corners with 4 different materials and see if the material used makes a difference.
An unknown in the example is the nature of the wall where it was hung. It could have been on a wooden wall and the craft paper dust cover could have little to do with the acid migration.
Like I said, just a wild guess.
How long ago do you think this was framed and what is the baking board made of?
15 - 20 years ago. Backing board is paper matboard.

Very slight discoloration around the edges (next to frame) otherwise pretty clean.
Thank you Wally... It must be the oxygen getting the the surfaces of the backing boards allowing the yellowing. I have taken apart fame jobs with slabs of wood behind them, and only place where there was acid burn was where the air could get through. My guess is air movement, and temperature changes.

Hugh? Can you shed any more light on this?
The wood would have kept the paper away from the white backing along its edges - the paper would not have been touching the backing there. But still there is a definite line.

So I still say it is light, maybe temperature too. Black absorbs light, black cars overheat before white ones which reflect the light/heat.

Acids react to light, leave some newspapers in a drawer, sure they'll go yellow. Leave them in the window, they'll go yellow quicker. Glaciers with rockfall melt faster because the dark coating absorbs the light, etc.

The wood protected the white backing from SOMETHING the paper could not, whether the paper was more acid-laden or not.
For what it's worth, I think the wood protected the paper from volatile acids, peroxides etc. emitted perhaps from the dustcover or oil based wall paint or whatever. The wood looks like mahogany (?) door skins. Used a lot in the 70' - 80's as backings for oversized art. Not as much adhesive and endgrain yech as plywoods or particleboard/chipboard. As Hugh has mentioned elsewhere it is the endgrain of wood that emits the most acid.

You see similar protection from the adhesive in brown paper tape (paper is discolored except under the tape) and oddly enough the adhesive in corrugated cardboard - the adhesive blocks pollutants from touching the paper, hence the striped acid burn from corrugated backings.

Andrew, sorry to come off so mystic...

As Rebecca said, those corners are 3-ply mahogany. Also refered to disparagingly as "Luan". Luan, is a tree in the Mahogany family that was commercially harvested every 20 years and rotor cut to make "door skins", and backing for "case goods". It was the "ply" that gave it the bad name I think as apposed to being a "lesser" Mahogany.

I don't know where you got the figures of acidic values for wood, but that spread looks suspiciously like the North American Softwood Association's numbers.

Hardwoods such as some maples and alder prefer a more alkiline soil and do not produce the toluines of the conifers. I have tested some maples, bass and alders in at 7.5 and 8; which is not neutral either but definately not acid.

As for the FACTS statement.... they don't like wood just because it isn't glass or rag.

Untill Don Pierce releases his strong hold and they commission seperate independent studies on acid content of which species and how and where it is transfered and which barriers are successful to what degree... they don't get my respect.

pH paper is cheap. Student labor is cheap [they work for grades and recognition]. I've gotten four art students to test what they are using and many of the supplies that we use.... my cost has been the occational pizza and beer. But I have numbers that help me be a better framer instead of just rattleing off knee-jerk diatrib "because it's always been that way".

I'm currently working on a needlework that was done in or around the 1790s. The last framing was done about 1860 (backing is newspaper. . .pre wood-pulp paper). The canvas is turned over alder stretcher bars that appear to be unsealed. The canvas shows no burning ONLY where it is touching the alder bars and the poplar frame.

Sorry to highjack your thread Rebecca. Andrew, maybe we will talk Friday when I'm down that way.

Thanks for the feedback ... very interesting stuff ... would love to learn more about pH levels of different varieties. I definitely agree that the more you know the better you can be.
By the way Baer the average pH levels that I quoted did not come from the N.A. association you mentioned but rather from a UK organization, TRADA (Timber Research and Development Association) ... and the figures mentioned related to their study of hardwoods. I wonder if soil composition in the UK influences this difference?

The pH levels quoted by TRADA are similar to another table that I have that specifically relates to plywood:

Problem with those tables is the lack of species or philum.

Ply-wood is a set of problems all to its own, because of the phenolresinals they use for bindery.

Mahog on the other hand responds better to PVA then PRNs so it was the bindery of choice for many years.
It was also used because of the skins being consumed more by the case goods industry then anything else.

Just about the only hardwoods that England has is Oak... and that is probable the two lines near the "3".
I remember having a list of PH levels in various timbers at one stage, they ranged into Alkaline for several species. I am looking for the list and will post it if I find it.
Backing boards in frames with strainers often show
the same sort of staining = discoloration of the
board, between the strainer members and less discoloration under the wood of the strainer. Side
grain of wood looks to be less polluting than
the air in many of our cities, and this may be what we see in that photo.


I understand that, but air is not restricted to straight lines, why did the pollutants not seep under the wood, at least some of the way?
The "cyclonic null barrier" in a confined space will cause the gas to stop permietation from projecting into the perscribed space.


They coner was laying directly on the board.
As Baer said, you need space to get air circulation. In exhibition cases that need atmospheric communication between the display area and a humidity buffering chamber, underneath, the minimum hole size is 1/4", to ensure the passage of air. Thermally insulated windows are engineered with such intervals in mind.

Just about the only hardwoods that England has is Oak...
quite right - oh and Apple, Ash, Beech, Birch, Box, Chestnut, Elm, Holly, Hornbeam, Horse Chestnut, Laburnum, Lime, Pear, Plane, Sycamore/Maple, Poplar, Walnut, Willow and Yew.

Paul Hardy
So, only theories then - has The Grumble finally been stumped!

I've only seen, or remember seeing, this sort of thing from the front - i.e. years ago I was sold some very cheap 'conservation' matboard, several suppliers were hawking it. The surface paper was garbage.

Framed some stuff with it, put it in my window, which used to get the sun for about 2 hours a day.

I put price tags on the glass (I know I know - that was then!)

The area behind the tags, on off white board kept its colour, the rest went darker, found same happened around the edge when the frame was opened.

No argument that that was light, but this one is from the back!
Tulip Poplar: So named because of it's Tulip shaped leaves.

John I think we know what happened, it's just the specific reative agents that remain unidentified.