Problems with Shellac Bubbling

F

Ford Fitzkee

Member
    I am having an issue with a recently applied shellac finish on a desk I built and was hoping to get some advice.  This is the same desk that had the potassium dichromate applied which I posted about earlier.  The problem I am having is tiny bubbles appearing on the top due to the heat generated by a laptop computer.  The shellac was fresh super blonde flakes, mixed in fresh denatured alcohol.  No problem with the shellac dissolving, I used a 1.5 # cut.  It was applied with a brush, with about 6-7 coats over the course of 2 days.  The shellac went on with no issues.  After allowing 24 hours to dry, it was rubbed out with #0000 steel wool to an even satin sheen, followed by an application of paste wax.  The rub out went great, with no initial problems.  The areas with the tiny bubbles are only under where the laptop was, so I have no doubt that was the culprit.  The bubbles are too small to photograph, but it feels rough like sandpaper.  The rest of the finish is having no issues.   
    My question is why the bubbles are forming.  The laptop is not getting extremely hot, probably 110-120 degrees max.  Good shellac is supposed to be heat resistant up to 150-160 degrees.  Is it because it hasn't fully cured?  Luckily the desk is for personal use and is easy to fix, but I want to prevent it from occurring again.  I'm thinking I may need a heat pad of some sort under the laptop to protect the desk top.  Or maybe once the desk has had a few weeks to cure it won't be a problem anymore.  Any suggestions would be appreciated.

-Ford
   
 
Ford, I have never used Potassium Dichromate, and I have never had an issue with a Laptop on Shellac. Have youinvestigated if there could be a possible reaction under the Shellac due to the heat?  Maybe a possible relative humitity reaction with the Pot. Dichromate created when the Laptop gets hot causing the Shellac to lift at the pores of the wood?
 
Hi Ford
I would do a few things different when applying shellac. First, I thin the first coat to about a pound cut to allow the shellac to bit in as well as it can. Next, apply no more than two coats a day, one in the morning and the other at night. More than that is like putting the lid on the shellac jar and retards the off gassing. They aren't really coats as each application melts into the last, but we'll call them coats. Anyway, thats where your problem started. After the sealer coats, change to about a two or 2 1/2 pound cut to start the build. Normally I start to French Polish after the sealer coat but if your brushing, only one coat a day if cool out side or you can't get about 12 hours in between applications. For brushing, three coats maybe four with light sanding in between should do it. Next, don't wax and no heat for about two weeks, wax can change things. You can re create the bubbles by placing it in the sun after application. You don't want to do that either. In short, you went to fast.
Randy
How about a photo to show what color you got?
 
Ford, I did some research about the Oxidation properties of Potassium Dichromate and it turns out that one of it's primary uses is for the oxidation of Alcohol using heat.  Chemistry was one of my required subjects in college, and although not immersed in the subject, had just enough to keep me curious about how things react.  Here is a university website discussion regarding oxidation of alcohol.  Remember, eventhough the alcohol evaporates during the application of Shellac, there is still residue left, likely in the form of alcohol sugars, which remain in the Shellac.  The Laptop heat source may have been the catalyst to activate a Oxidation Process.  Just a thought!

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alcohol2.htm
 
I definitely applied the coats quickly, sometimes only waiting only an hour for the shellac to be dry to the touch between coats.  This recommendation is common, with some people applying 4-5 coats in a single day.  I now see the potential problems with that approach.  The shellac really needs more time to fully dry, despite being dry to the touch so quickly.  I removed the wax from the top with mineral spirits and sanded the top smooth.  I will rub out again with steel wool and let it cure for a few weeks before putting it to use.  I will probably use a heat pad to protect the desk from the laptop until I'm confident it has fully cured.

That's interesting info about the potassium dichromate. If there was a potential reaction with the alcohol from the shellac and the PD, I would have expected to see the issue while the shellac was drying.  The heat from the laptop was fairly mild, just warm to the touch.  Also, I'm pretty sure the PD fully reacted with the tannic acid in the wood, leaving nothing else to react with it.  I am not a chemist so these are just guesses.

I really appreciate the help on this issue. Below is a photo of the desk after the PD was applied with super blonde shellac.  I am very happy with the result.  I also posted a follow-up report on my experience with the PD on the Darkening/Oxidizing Mahogany post  http://www.sapfm.org/forum/index.php?topic=2526.0

-Ford
 
Ford, you presented a great mystery.  I had some time between projects to focus my attention concerning this.  Potassium Dichromate is used to convert alcohol to Ketone through Oxidation using heat.  Natural Tannin is an organic form of Sulfuric Acid, and tannin has been used throughout the centuries to Tan Hides.  Ketone in part, is a paint stripper.  What seems to have happened is the Tannin, in solution with Potassium Dichromate, with the application of heat was converting the Alcohol used to apply the Shellac into a solution consisiting of Ketone and other properties and working to lift your finish...as in stripping it!

More info:  http://www.pearsonschoolsandfecolleges.co.uk/Secondary/Scotland/Science/ASandA2OCRBiology/Samples/A2StretchandChallengeSamplePages/Sp_1_1_10.pdf

I have a neighbor who recently had a house fire caused by a Laptop Computer.  Extensive damage!!  the Laptop was on an antique table and left on while they went to work.  The insurance company thinks it may be a battery issue...now you have me wondering! 
 
    This is certainly interesting and I really appreciate the research into this matter.  I am inclined to believe, however, that the problem is being caused by the shellac not being fully cured based on a test I did today using the finish samples I made up for the desk project last week.  I had some samples with various concentrations of potassium dichromate used and some samples without the PD.  All had 5 coats of 1.5# shellac and were completed last week, so they have had about a week to cure.  I used a heat gun on a low setting to simulate the heat caused by the laptop.  The heat was well below the temperature that would melt the shellac.  I heated the samples so they were warm to the touch, like the table felt with the laptop running.  I was able to get the same effect that I got on the table for all my samples both with the PD and without the PD.  The only sample I had a hard time to get the same effect was the first sample I made over 2 weeks ago (No PD).  I eventually got the bubbles to form but I had to get it really hot and I think I basically crazed the shellac, so not really the same effect.  Other samples without the PD that were made last week got the effect with very little heat from the gun. 
    So I'm fairly certain it's the cure time, not the PD. Keep in mind that the shellac is not being stripped off, just tiny bubbles that are too small to see but basically feel rough like sandpaper.  The good news is that #0000 steel wool takes the roughness away, making it silky smooth again and there is no visible damage.  So if it happens again it can be repaired very easily with out any additional shellac applied.
 
I doubt it has anyting to do with the pot. dichromate...There was air trapped in the pores of the wood.  The laptop created heat that expanded the air bubbles and they came to the surface.... Thin your shellac more to start....It is easily repairable though.
 
CBWW said:
I doubt it has anyting to do with the pot. dichromate...There was air trapped in the pores of the wood.  The laptop created heat that expanded the air bubbles and they came to the surface.
Click to expand...
I would agree; it sounds like a heat related issue and nothing to do with the use of potassium dichromate.
 
Ford,

I can tell you about two different occasions when I delivered the furniture right after finishing on a hot day.  One did use some P. D., the other did not.  They both had the bubbling you describe.  I made profuse apologies and back to shop they went.  I think,  like you and others, that it is a case of heat causing alcohol in the pores that has not flashed off to expand.

That is a very nice desk and your clean, neat shop makes me hang my head in shame.  PSP
 
Ford
I'm impressed with your testing, not many take the time. When I give classes on shellac and French Polishing I have the students create problems intentionally. Fixing and knowing why it happen is the first step in prevention. Not allowing time for off gassing is a common mistake. Another problem with going too fast is shrinkage.  Half a dozen coats in a couple days looks good when it leaves the shop but as it dries it will shrink and can expose pores, especially in pores wood like oak.  Also delivering freshly finished surfaces too soon can get press marks.  A vase or something heavy can sink into the shellac if applied too thick or time isn't allowed for setting up. When I worked antique shows a common problem was packing marks from restorers finishing something before packing it into trucks. The piece would arrive at it's destination  with a nice quilt pattern in the finish. More time for drying and fewer and/or thinner coats. Try French Polishing, thinner finish but beautiful. Also opens up a world of repair techniques not available with other finishes.
Regards
Randy
 
Thanks to everyone for the input.  It's definitely a lesson learned on my part.  Rushing the finish in this case ended up costing me a lot more time in the end.  I took the heat gun to the samples again yesterday, a day after first using the heat gun to cause the bubbles.  No more bubbles or roughness appeared, so the initial heat caused all of the residual alcohol and whatever else was trapped to off-gas.  It looks like the heat gun on a low setting can significantly speed up the curing process.  Probably not the best technique, but for a piece that is going to have something that produces heat on it (like a laptop), it may not be a bad idea to very carefully use a heat gun on a low setting to off-gas any residual alcohol prior to rubbing out, just as an insurance policy.  Of course, proper technique and curing time probably make this unnecessary.

-Ford
 
Jeff,
  The only thing applied to the surface before the shellac was potassium dichromate to oxidize and darken the mahogany.  No oil was used, just shellac.  The shellac finish now has no issues with heat from the laptops or anything else. I think it just wasn't fully cured and there was still some gas trapped. This issue is definitely a factor for anything that will have electronics on it like a computer or even a cable box. I'm glad it was something I built for myself and not a customer.

Ford
 
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