Chemically stained Sapele desktop final outcome

[Prashun Patel]

Very nice. I am interested in your staining process. Can you pm me the steps or post them here (or provide a link if you've already posted?) The reactive stains would appeal to me in some cases where I'm hand applying a top coat.

[David Hawxhurst]

I'm not familiar with WB.

WB is waterborne. he was using general finishes conversion varnish. they have managed to make a catalyzed wb finish.

[scott vroom]

Really nice look Scott. That process sure did work nicely. Congrats! By the way, does that finish provide a hard surface? Is it apllied via spray or ??

Hi Pat..thanks! The General Finishes conversion varnish is advertised as a durable commercial grade varnish suitable for bar tops, restaurant tables, etc. I called GF tech support and was told that the CV was tougher than their Enduro poly, which I've used for similar projects in the past. It fully cures in 1 week...been 6 days and it seems rock hard. Time will tell. I sprayed with a Fuji 4 stage HVLP with a 1.4MM tip. It was my first attempt with a conversion varnish and I found this one a bit more difficult to work with than poly...it seemed to want to go on thicker to get an even looking coat....I guess I'd say it was a bit less forgiving than poly.

[scott vroom]

Looks great! The overall tone is nice and even with the ribbon figure coming through nicely. Can you spell out the finish that you used? I'm not familiar with WB.

Ribbon sapele is one of my favorites. It is denser and more stable than ribbon khaya, although the ribbon shimmer is not quite as pronounced. And sapele smells a lot better when you cut it. Sharp tooling is a must to avoid tearout from the interlocked grain, but we get much better yield from sapele compared to khaya with carbide insert heads.

Thanks JR, I used General Finishes waterborne conversion varnish. I've never worked with ribbon sapele and had read about tear out from the alternating/interlocking grain. I used my 15" Grizz planer with a spiral cutter.....the only tear out I had was one small area near the end of the board which ended up getting trimmed anyway....I guess I was lucky in that regard.

The wood was quite stable....I believe ribbon grain is quarter sawn. Boards from the mill were flat...I face jointed them just to be sure but probably could have skipped this step. Really nice wood to work with.

[scott vroom]

Very nice. I am interested in your staining process. Can you pm me the steps or post them here (or provide a link if you've already posted?) The reactive stains would appeal to me in some cases where I'm hand applying a top coat.

Hi Prashun,

The potassium dichromate comes in a crystalline form....looks like ground orange glass. It dissolves quickly in warm water; I tried varying strengths on sample boards before settling on a ratio of 1/2 teaspoon to a pint of water. First step was to pre-raise the grain by sponging a copious amount of clean water on the boards, working it into the grain before wiping dry. I sanded the fuzz smooth with 320 grit on a ROS. Next I applied the chromate solution with a cheap 4" chip brush, using long overlapping strokes. Next I used a wad of paper towel to work it into the grain with a swirling motion for a minute or 2 then wiped off the excess. The chromate works fairly quickly. As the chromate dries the wood takes on a muted, flat appearance, having lost much of the ribbon grain contrast....so much so that I thought I had screwed it up. I let it dry overnight then rubbed on some clear poly on the end as a test....wow the color and grain pop was dramatic. Similar to what I've read occurs to ammonia fumed oak....looks gray and flat after fuming but comes alive with a topcoat. 6 top coats of conversion varnish were applied via HVLP. I'm pleased with the outcome and will use the PD again on another of the high tannin woods. Apparently it's popular on cherry, where instant aging/darkening is desirable.

As another poster pointed out, potassium dichromate is toxic and must be handled with common sense care. I wore a vapor respirator, rubber gloves and eye protection at all times, from mixing to final wipe down. The crystalline form makes it less prone to becoming airborne in it's dry state, but one must be careful if it becomes necessary to sand a treated surface.

[Prashun Patel]

Thanks Scott. Does the piece require rinsing after the reaction's complete?

[Frank Drew]

Excellent selection of boards and grain matching, Scott.

I'm in the okay-with-potassium-bichromate-used-sensibly camp, particularly at the level of exposure those of us here would experience; I've only used it with Mahogany but never noticed any issues with fading.

[scott vroom]

Thanks Scott. Does the piece require rinsing after the reaction's complete?

Prashun, no rinsing required after the reaction is complete. Assuming you pre-raised and sanded prior to treatment, the surface is ready for top coating once the treated surface has dried (shellac.net recommends 12 hours dry time, however in a warm shop I'm sure the drying time would be considerably shorter).

The chromate oxidation doesn't penetrate deeply....I tested this by carefully sanding off a small test section. For this reason (and the obvious safety reason of avoiding putting treated dust into the atmosphere), I built up several deep coats of varnish before I attempted a first sanding. The chromate reaction is unlike ammonia fuming, in which the effect can penetrate 1/16" or deeper into the wood surface. This is why it's critical to pre-raise and sand the grain prior to treatment.

[bill tindall]

At the time I did the testing I had access to accelerated weathering equipment that provided conditions of controlled humidity and light source. The experiment consisted of staining sequential pieces of genuine mahogany with potassium dichromate solution, sodium hydroxide solution, ammonia vapor and a mahogany dye stain mixture recommended by Fred Stanley, and Alf Sharp recent SAPFM Cartouch winners who I regard as experienced experts in finishing mahogany.

There are a number of chemistries used to make soluble dye stains- azo, anthraquinone, and a more modern category called "premetallized". The later show exceptional lightfastness in many situations compared to the older dye chemistries. Many of the Moser dye stains are premetallized while some of the other sources are not. The dye stain MSDS will usually reveal its chemical family.

Half the sample was masked and the samples exposed to simulate daylight under some typical humidity which I don't recall. The ammonia and sodium hydorxide colors were fleeting, that is they showed poor lightfastness. The dichromate showed good lightfastness but it was significantly poorer than the premetallized dye.

The conclusion is that there is no reason to need to resort to potassium dichromate to color mahogany or walnut.

It seemed possible that once potassium dichromate was applied to wood that the harmful Cr(VI) form would be converted to relatively harmless Cr(III) by the color forming reaction. No doubt some was, but I was able to extract Cr(VI) from the wood after staining.

[J.R. Rutter]

WB is waterborne. he was using general finishes conversion varnish. they have managed to make a catalyzed wb finish.

Thanks. I knew what WB meant but not GF.

[scott vroom]

At the time I did the testing I had access to accelerated weathering equipment that provided conditions of controlled humidity and light source. The experiment consisted of staining sequential pieces of genuine mahogany with potassium dichromate solution, sodium hydroxide solution, ammonia vapor and a mahogany dye stain mixture recommended by Fred Stanley, and Alf Sharp recent SAPFM Cartouch winners who I regard as experienced experts in finishing mahogany.

There are a number of chemistries used to make soluble dye stains- azo, anthraquinone, and a more modern category called "premetallized". The later show exceptional lightfastness in many situations compared to the older dye chemistries. Many of the Moser dye stains are premetallized while some of the other sources are not. The dye stain MSDS will usually reveal its chemical family.

Half the sample was masked and the samples exposed to simulate daylight under some typical humidity which I don't recall. The ammonia and sodium hydorxide colors were fleeting, that is they showed poor lightfastness. The dichromate showed good lightfastness but it was significantly poorer than the premetallized dye.

The conclusion is that there is no reason to need to resort to potassium dichromate to color mahogany or walnut.

It seemed possible that once potassium dichromate was applied to wood that the harmful Cr(VI) form would be converted to relatively harmless Cr(III) by the color forming reaction. No doubt some was, but I was able to extract Cr(VI) from the wood after staining.

Hi Bill,

I'm not a chemist so much of the jargon is lost on me. TransTint die is advertised as "a single component concentrated metallized acid dyestuff dissolved in a glycol ether carrier." Would this fall under your definition of a pre-metalized dye? If not, what commercially available brands would you recommend? I'm entering a phase in my woodworking where I'm interested in experimenting with new (for me) finishing products and techniques. I agree the PD requires added safety measures and is not suited for someone not comfortable with safe handling practices.

[Jamie Buxton]

Scott, I've been reading about real conversion varnish, like the General Finishes product you used. I find lots of cautions about isocyanates, which are produced in the spraying and curing process. Like you, I'm not chemist enough to know what isocyanates really are, but OSHA seems very concerned about exposure to them. That's why autobody painters now wear hoods with air supplied from outside the booth. What's your take on small-woodshop use?

[scott vroom]

Scott, I've been reading about real conversion varnish, like the General Finishes product you used. I find lots of cautions about isocyanates, which are produced in the spraying and curing process. Like you, I'm not chemist enough to know what isocyanates really are, but OSHA seems very concerned about exposure to them. That's why autobody painters now wear hoods with air supplied from outside the booth. What's your take on small-woodshop use?

Thanks Jamie...I'll look into it. I do wear a vapor mask when spraying and also pull air through the shop via a high velocity exhaust fan.

[bill tindall]

Would this fall under your definition of a pre-metalized dye? ...yes The Moser dyes (available from Woodworker Supply and elsewhere)I investigated were premetallized (or metallized). There is some classic source in NY (name not remembered) and the dyes from them that I checked were not metallized.

That said, there are probably 100+ chemicals out there used to make wood dye stains. I have no doubt that they vary in lightfastness and I have only checked two. But as a general rule the premetalized dyes are more lightfast than the typical azo or anthraquinone (so called "aniline") dyes.

[bill tindall]

Isocyanates are a family of chemicals used to crosslink, and therefore harden and toughen, film finishes. They vary a lot in toxicity and ability to trigger an allergic reaction. Because they harden and toughen they are desirable in a finish that is going to experience daily use, get washed, have alcohol spilled on it, the cat use it for napping, etc. But there are all sorts of chemicals in the finish that you don't want in your lungs. Hence, a chemical respirator is recommended for any spray finish.

Conversion varnish is my preferred finish. It is tough, hard and looks good. Thinned it can simulate an oiled finish or built heavy it can be rubbed out to a high gloss mirror. I am on my 4th- 5 gallon can of it (Sherwin William Kemvar M). But even at this usage I am exposed to but a tiny fraction of the spray as someone spraying professionally for an 8 hour day. My personal philosophy is anything approved to be used professionally on a daily basis presents no practical hazard for my limited use so long as I take reasonable precautions. After spraying conversion varnish I let it cure in a well ventilated space, for it does stink for a day or two.