Some people asked how did I get the deep, nice finish on my latest cherry projects. Some of you might know the answer: potassium dichromate. Since the information I see on the web about the use of this “magic substance” is inconsistent, I’ll try to give you my point of view and experience.
WARNING: I am only offering my opinion without any implicit or explicit liability or fitness to a particular purpose. Use this advice at your own risk; I do not assume any responsibility.
Note 1: As questions are asked, I’ll be adding to the F.A.Q. at the end to build more complete knowledge.
Note 2: I added a comparison with lye at the end since it is the most used other substance for same effect.
Before we look into potassium dicromate (I’ll call it PD for short), let’s first ask the question: why old cherry is so much nicer than the new cherry. It is not only the color but the depth and richness of the finish. Well, there is no secret really: most woods contain tannin. When tannin oxidizes, it colors the wood. For example, when tannin reacts with the iron in the presence of water, you get black/blue streaks in the wood (that is how you recognize nails in the wood).
How does the tannin oxidize? When the wood gets exposed to Ultra Violet light (always present outdoors and present enough indoors unless filtered), the UV rays energize the watter particles and unbond the oxygen from the hydrogen. The free oxygen combines with the tannin and color the wood. The process is usually very slow but on some woods, a good day of exposure to sun can produce visible results.
What is potassium dicromate? Without getting into a lot of chemistry, potassium dichromate is a substance that has a lot of loosely bound oxygen. In fact, it is ready to give up that oxygen at the first chance. When in contact with substances that can oxidize (combine with oxygen), potassium dicromate looses the oxygen. Now, most organic substances oxidize, so they will react with the potassium dicromate, including your hands.
Wood Samples: Each sample finished half with and half without PD followed by a coat of shellac (brushed).
A. Cherry. The PD darkened the cherry a lot and brought out the gorgeous grain
B. Pine. Notice the deeper but yellowish color
C. Pecan, sapwood. Yellowing
D. Pecan, heartwood. Nice subtle caramel color. Brings the grain and color up.
E. PD in the jar. Crystals are quite big (looks like rough salt). There is no fine dust so no need to get paranoid.
F. Handle with care. Wear vinyl gloves.
Frequently Asked Questions:
1. Does PD color the wood? No, at least not in a significant way. PD has a faint pinkish color (see above pictures). What PD does, though, is it oxidizes tannin and colors the wood in 1/2 hour as much as years of exposure to light.
2. If you apply more, do you get a deeper color? When PD fully reacts to the tannin on the wood surface it has no effect on the color. You can put as much as you want but the color will stay the same.
3. How hard is it to apply consistently (i.e. get a nice uniform color)? Very easy. Since the wood cannot be colored beyond the saturation point, you just want to make sure you cover the surface well. You can apply it twice to make sure you did not miss any spot. I apply PD with a foam brush (that I keep recycling until destroyed) by basically brushing it on from a plastic container. You wan to make sure you do not splash around,especially on your hands or feet.
4. How does the PD solution get prepared? I put a small quantity (1/4 tsp at most) in 1 pint of warm watter and stir until dissolved. That is it. DO NOT DISSOLVE A LOT. It is not needed and the solution is unnecessarily dangerous.
5. At what stage of the finishing process should I use PD? I usually scrape then sand to 220 the piece. I apply PD at this stage.
6. How much do I have to wait to start finishing? Until the surface is dry, usually 1 hour. I sand lightly afterwards with 220 grit since the watter in the solution raised the grain (a good thing since it raises anyway latter).
7. Will it color the sapwood? NO. It will age the sapwood making it look yellowish but not darken it significantly. The sapwood does not have tannin.
8. Does it make finishing with some types of finish more difficult? Watter and residual PD on the wood are evaporated or sanded away. What is left is just the wood with the tannin oxidized and the wood behaves the as if it natural. No finishing product will react with the color. No restrictions since it is not a stain nor a dye.
9. I’ve head it is a nasty substance. Is it? PD has a bad reputation because the chemical manufacturers have to put strict warnings. If you have to shovel a truckload of PD, then yes, be concerned, very concerned. If it gets into your nose, it produces severe inflammation, if it gets on your skin (as a solution) it produces burning. Now, if you want to use if for furniture, all you need is basic precautions: a. Wear a respirator when you mix the powder with the watter. Once in the water, there is no danger to your respiratory system. Even dust after the solution dries is inert since the PD reacted with the wood (it reacts with everything except plastic). b. Wear gloves when you brush it on. Latex gloves are no good since they are permeable to small molecules. Vinyl gloves are perfect c. If you splash yourself on hands or legs, go wash in a lot of watter. PD in solution is about as nasty (or not) as chlorine (bleach). Try not to splash yourself in the eyes. If you do, wash for 15-20 minutes (same as chlorine). d. DO NOT DRINK IT. more importantly, keep it away from children and do not keep it in beverage containers since it can look a little like soda.
10. On what woods should I use it? Well, experiment. Above you see it used on 3 types of wood with varying millage. On cherry, it is always exceptional and so is on mahogany. I used in on pecan and it produces a nice, subtle effect. On pine it is not so nice and on oak it does almost nothing.
11. Does it really keep the original wood color? I have compared pieces finished with PD and 5 year old pieces that were not but got exposed to light. I see no difference beyond the normal color variation you see in cherry.
12. Is it expensive? Where can I get it? You can buy PD from ebay or chemical supply stores for about 10$/lbs . 1lbs is enough for a lifetime so do yourself a favor and buy only 1 lb and share it with friends.
13. Can I apply it on veneer? Yes. As long as you apply it on wood, it will do its magic. On plastic it does nothing. It stains everything else.
14. Can other substances oxidize like potassium dicromate? In principle, yes but they might have other undesirable effects. Potassium permanganate, for example, is a lot more reactive, thus riskier to work with.
A mild solution of PD is a lot better; it oxidizes in 1/2 hour and you really do not want faster oxidation. Also, once the tannin is fully oxidized, there is no more coloring. You will not get a deeper color with a nastier substance.
15. What is the proper procedure to dispose of PD?(thanks sras) I usually do not dispose of the solution since it does not go bad in time. I just keep it on the shelf and use it at a latter time. Proper disposal procedure is probably similar to other nasty substances: take it to the recycling center.
16. Does it really have no effect on oak?(Dusty56) To be honest on this one, I would have to experiment again but as far as I remember, the effect is subtle to no-nexistant so I prefer not to use it. I might be wrong, hence the experiment.
17. Is glass OK for storage or just plastic? (Pawky) Glass is perfectly fine for PD (I store the crystals in a glass jar). Interestingly, glass is not OK for lye since it will slowly eat through it.
18. Do you need to scrape and sand for PD to work?(Pawky) Nothing special needs to be done for PD to work. My normal finishing routine consists in scraping followed by 220 sanding but I do that for reasons of speed and quality of result not for PD to work. Any finishing procedure is fine.
19. Do you need to use distiled watter with PD? (BigTiny I do not know but I suspect not (see section below). PD does not seem to be so reactive as lye. For example, I put PD on iron and aluminum and I got no reaction. If you have distiled water around, it never hearts to use it, though.
20. How deep does the color penetrate the wood?(Pawky) The effect of PD (and probably lye) is dependent on watter penetration, which is not much for side-grain. My guess is that the coloration only goes inside for less than 1/256 in (superficial). Sanding more than lightly will definitely discolor it (but then you can put more PD).
Comparison with lye (sodium hydroxyde or caustic soda)
Lye keeps on popping out among the substances used by other LJs so I took the trouble to look into it. Here is what I found in the form of advantages/disadvantages:
Advantages of lye
1. More available, especially if you are impatient. PD has to be bought from the web but lye is available locally.
Lye is probably cheaper as well.
2. Reacts faster with the wood.
Disadvantages of lye
1. It is a nasty substance. The Wikipedia page has a picture of a severely burned hand. Chemical gloves and full on protection is probably needed. With PD, I just wear cheap vinyl gloves and when some makes it on my skin, just flush with water. I never get burns (just mild irritation).
2. Lye reacts with everything. Distiled watter seems to be strictly needed since lye will react with residual metals and get various non-desirable side-products. I was surprised to see that it even reacts with glass (eats it away) and that it absorbs humidity. This means it goes bad, so it is irrelevant if it is cheaper than PD. Keep in mind that the 10-15$ investment in 1 lb of PD is expected to last many years since minute quantities of PD are needed even for large projects. My entire king size bed required less than 1/4 tsp of PD.
3. Lye seems a handful to handle and store properly. Cannot be stored in glass. When in contact with aluminum, produces hydrogen and can get explosive, reacts with everything violently (hands included). Any of these highly corrosive substances can produce harmful fumes. When lye was mentioned, I was tempted to volunteer to compare it with PD but after reading about it, I’m not tempted at all. I would personally not deal with it.
All in all, to me the disadvantages overshadow the advantages. Price and reaction time are not so important (I have to wait for 1 hour for the water to evaporate anyway, it does not mater if full color is achieved in 1/2 hour or instantly). Danger of manipulation is important to me so I would rather not use it. I think historically lye was the first chemical to be used for this purpose since it is easier to synthesize. PD is more modern and has been used extensively only in photography (film development) but it is a much nicer substance. I’ll keep on using PD and not even try lye.
Thanks for reading. I would appreciate feedback on the quality and usefulness of the above information (so I can do better next time).
-- -- Alin Dobra, Gainesville, Florida