50th Wedding Anniversary Box & "Shellac Bubbles"

[Mike Scoggins]

I recently completed a box for my parents 50th Wedding Anniversary (their anniversary is actually today). We will be celebrating the event on June 5th at which time I'll give it to them as a gift. It will be used to keep cards and letters of congratulations and the sign-in/registry for the event.

The box's outermost dimensions (i.e. length and width are that of the base; height is the box with the base and lid) are 18" x 13" x 8-1/4". The box is constructed of cherry with through dovetails (my first). The base and the frame of the lid is maple. The raised panel in the lid is cherry. The lid frame has mitered corners and cherry splines. The finish is about 5 or 6 coats of dewaxed blonde shellac. As with all of my projects, there are plenty of mistakes, but I also learned several things and attempted some firsts. This was my first project with dovetails (I bought the Leigh dovetail jig at the Houston Woodworking Show this year) and my first use of shellac.

After the box sat for about 2 weeks after the last coat of shellac, I decided to set it out in the sun on a Saturday when I was working in the garage/shop to hopefully let the sun do its magic in darkening the cherry. The sun did its trick, but I encountered something I didn't expect. For lack of a better term, "shellac bubbles" (hard, not soft) appeared on the ends of the raised panel in the lid. I have a picture below that shows these bubbles. I suspect that the shellac did such a good job of sealing the pores on the end grain of the panel that air was trapped in the wood and that the heating of the sun caused the pressure to increase thus forming the bubbles. Is my suspicion correct? Has anyone else ever encountered this phenomenon? Is there a way to prevent this? (FYI, I padded on 3 coats of shellac on the raised panel before assembling the lid. There are about three coats of shellac that were applied via spraying on top of that.)

I will be sanding the bubbles away and, if necessary, applying another coat of shellac to remedy the problem.

Thanks for looking and for any insight on the "shellac bubbles!"

Mike

Note: I had problems with the pictures uploading in the original post.

[Mike Scoggins]

"A picture's worth a thousand words," and maybe even more on SMC! I edited my original post so that the pictures are now included. (I had problems with the attachments uploading originally. :mad: )

Mike

[Rob Russell]

That's a beautiful box. I wouldn't be surprised if there's a wet eye or 2 when they see it.

I have no advice on the shellac bubbles - I've never used shellac. I'd guess trapped moisture that vaporized in the sun, but that's a SWAG.

[Dick Parr]

Very nice Mike. How come no pictures of the top? :confused: From what little I can see it looks interesting. :p

[Todd Burch]

Mike, my first thought was that cherry sap has come out or the end grain of the fillet in the raised panel and hardened, but I'm not sure.

I have a friend that is the local rep for Zinsser. I've emailed him the photo and your descriptive information. Let's see what he or his lab guys suggest.

[Todd Burch]

By the way Mike, and any other local woodworkers - I've talked this shellac rep into giving a presentation at the August Woodworker's Club of Houston meeting. (8/14/2004) A lady from Corporate is even flying in to talk to us and educate us about shellac!! A good opportunity to learn. (See www.wwch.org for info about the club)

[Mike Scoggins]

Rob,

Thanks for the compliment.

Dick,

Thanks for the comments. I took several pictures of the box last night and all the ones for the top were blurry (didn't realize the top was not included until you pointed it out). I'll see if I can't get an acceptable shot and post it some time this weekend.

Todd,

Hmmm -- hadn't thought about the possibility of sap. Just based on it's appearance and feel, I don't think its sap, because the "bubbles" seem more delicate (thin-skinned) than I think sap would be, but I'll probably find out real quick when I attempt to sand it this weekend. Thanks for the extra effort in contacting Zinsser and for the information about the upcoming presentation on shellac at the August meeting of the Woodworker's Club of Houston. I've thought about going to one of the meetings before, but have never done so. Maybe I'll be able to on 8/14.

Mike

[Tyler Howell]

Very tastie Mike, Looks good enough to eat.;)

[Byron Trantham]

Mike, nice looking box. I use shellac a lot and have not run into this problem. However, shellac will bubble if you were using a brush to apply it. I use a brush and keep an eye on "where I have been". If I see bubbles :eek: , I just add a bit more shellac to the brush and use the "tipping" technique to "pop them." :cool: Tipping a brush is moving it over the surface at almost a 90 degree angle "just barely touching the surface." This generally pops any bubbles that might have formed.

[Mike Scoggins]

Byron,

Thanks for your insight. However, the problem I had in this case was/is not with the application. When I finished the last coat (which was sprayed), the finish was silky smooth. There were no "bubbles" at all until I let the box sun tan for a day.

Mike

[Byron Trantham]

Ok, I see. I guess the food news is shellac is so forgiving. Sand it and re-coat. I imagine the sun has "cooked" all the stuff out. :D

[Brad Schmid]

Hey Mike, glad you posted the pics! Turned out real nice.

I have a guess on what happened...
That cherry was probably about 10-12% moisture content. Of course it was then sealed in by the multiple coats of shellac. I'm guessing sitting it out in the sun may have made the immediate surface of the top hot enough to expand that moisture through the end grain near the surface.

I'll be interested to see what Todd turns up with th Zinsser rep.

Again, beautiful job!

Brad

[Mike Scoggins]

Brad,

Thanks for the comments. Your explanation makes a lot of sense to me. And yes, it will interesting to see if the Zinsser rep has additional information that may be helpful.

By the way, it's good to see that you've been able to get back out in your shop some this week. Hope your recovery is ahead of schedule.

Mike

[Brad Schmid]

Mike,
recovery going well! from crutches to cane pretty soon. yahooo!

I'm trying hard to stay away from the lathe long enough to try and finish up the dresser that you know has been in progress for at least a few months :D

BTW - I can bring the moisture meter over and we can see what the moisture content is... It's pinless, so it won't harm the finish. We can also check a leftover unfinished scrap and compare to see if it gives any clues.

Brad

[Todd Burch]

Hi Mike. I got a reply back from Zinnsser. This is from Gene Hoyas, the International Product Manager for shellac based products - the same person that will presenting at the WWCH meeting on Aug. 14th:



"It's shellac. From what I can see in the photo, it is being forced out of the fillet - possibly as the result of softening and expansion due to minute amounts of solvent trapped in the initial or second coats. If he applied several, very heavy coats in rapid succession you have your answer. Hope this helps."

Todd

[Bob Hyde]

Mike, very nicely done... great detail and very easy to look at! While I don't spray shellac very often, I use a fair bit of it. The bubbles you showed were foreign to me until I saw Todd's follow-up. Sure is slick the way this gang helps each other out... I'm looking forward to being part of it.

best, bob

[Mike Scoggins]

Bob,

Thanks for the kind words.

Todd,

Thanks for your extra effort in contacting Zinsser and posting their response. Sounds like that may well be what happened. Makes sense that there was probably some solvent trapped. The shellac had to be softened either by solvent, heat or the combination of the two.

My good neighbor (Brad Schmid) came over on Saturday and measured the moisture content in the box and then compared those readings to those from the source pieces. Thanks Brad! ;) In general, the moisture content in the box lid panel (~10.5%) was a full percentage point less than in the box sides/ends (~11.5%+). So, the heat of the sun apparently "dried out" the panel more than the sides since the panel was horizontal and as such the surface area of it was more efficiently heated by the sun.

So, with the information from Zinsser and the moisture content data, I suspect the combination of these factors resulted in the bubbles. I'm thinking that there probably was some alcohol trapped in the initial finish and the heating from the sun expanded the moisture/air which essentially blew (formed) the bubbles. I don't know how high the temperature would have to be to soften shellac sufficiently for the bubbles to form, but: (1) the temperature reached that point from the sun's heating, (2) there was sufficient solvent trapped to allow such, or (3) the combination of the two created conducive conditions (say that 3 times fast) for bubble forming.

At any rate, thanks to everyone who provided comments and ideas. This is a great site because of just such helpful insight.


Have a great Memorial Day!

Mike