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Light box question


jackc
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When I open my light box and remove a violin being varnished, it's quite warm inside, maybe 110-120 degrees F. Is this good or bad in terms of drying speed? The heat results from the closed box having no ventilation.

It would on the one hand seem that an elevated temperature would reduce drying time. On the other hand, the vapor pressure (if I'm saying this right) would be increased by the closed box such that any volatiles, like the turp in the varnish, would evaporate slowly, as the air in the box would quickly be saturated with vapor.

OK, it's clear that I don't really know what I'm talking about here, which is why I'm asking the question: Should I have some air moving through the light box?

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Yes I'd say ventilation, movement of air and keeping the temperature from getting too high are a good thing for a drying cabinet ............ I also keep a couple of bowls of water in mine with the vague notion that I don't want it to get to dry in there.

neil

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Heat does speed drying of oil varnish (Fry did tests on this which are documented in his book,but his varnish is troublesome to dry so i dont know how accurate his results really are)but id also recommend some sort of extracter fan in the top of the box.

Nertz idea of water in a bowl is good to reduce any movement in the wood .But be careful with heat and moisture or you could have open seams ,etc...

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According to H.S. Wake's "A Luthier's Scrap Book", he describes that the temperature should be at about 90 °F (32 °C). The temperature is said to be needed for proper curing of the varnish. In my opinion this is far too low to be effective as curing temperature. One would need at least 140 °F (60°C) to have any curing effect.

To maintain the temperature he (H.S. Wake) keeps the door a little bit open. Also a thermometer with outside reading would be useful.

I don't have a drying cabinet [because I used to varnish with spirit varnish] but when I should make one, I would built in a small ventilator that transfers the warm air from one side to the other. The draught is created by two holes in the cabinet.

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Mine has a small computer muffin fan that pulls in air through two large filters at the bottom of the box. The air is forced out through holes at the top. The box is warm probably 85F. I am very happy with it. I believe you can see it in the photos of my workshop in another thread. Notice that it has a UV absorbing window. The hanging violin rotates twice a minute.

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Is the UV absorbing window needed because glass does hardly transmit UV radiation. Placing air filters is very handy to avoid dust particles but how does this relate to the resistance to pass the air through the filters?Too wide pores do not filter off small particles. If you have PC fans thye are not so powerfull to pull air through narrow filters.

Maybe I'm wrong, but in stead of rotating the instrument could placing a mirror or even alu foil already be sufficient to spread the UV radiation?

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quote:


Originally posted by:
jackc

When I open my light box and remove a violin being varnished, it's quite warm inside, maybe 110-120 degrees F. Is this good or bad in terms of drying speed? The heat results from the closed box having no ventilation.

I have never used ventilation. There are enough cracks for ventilation. The temperature has never been a problem for me, and in fact I have a couple of 75 watt incandescents also to keep it warm in the winter. I use only one 30-watt germicical lamp. (The others burnt out and I never replaced them; it works fine.)

UV dries varnish, but how? I think it is the ozone in small quantities that is the reactive component. Disallowing ventilation might actually help dry the varnish.

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A UV lamp may be a source of ozone - I can't speak to that - but my impression is that UV catalyses some reactions. I assume the thrust of the lightbox concept is to more or less speed up the process of drying the varnish, while protecting the surface from airborne particulates.

I would question the use of UV insofar as it would not have been a major element in the varnishing of, say, Cremonese instruments, which are pretty much the gold standard in violin luthiery. Whatever chemical reactions it may produce in the drying varnish would not have taken place, at least to the same extent, in a fiddle hanging in the sun.

There would have been little exposure to ozone back then, either, with no source of electricity to make it other than the occasional thunderstorm.

All that said, has anyone done any controlled experiments with this type setup? It'd be interesting to compare blocks of wood, varnished with the same compounds at the same time, dried in a box and the open air, with an eye toward the nature of the final product.

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My light box has two UV lights on front and back (4 total) and is lined with aluminum foil. I read somewhere to keep the temperature below 100 deg., so there is a thermostat to keep the temperature between 80-100 deg., and the lights aren't continuously on. The box is made of 1/4" plywood so the heat isn't trapped inside and the lights regularly cycle on and off. This set-up wouldn't work with as well with an insulated box. It dries oil varnish in 24 hours. There is no ventilation (except small cracks around the door) and no water is used. This set-up has worked well with no problems.

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Johnmasters writes.....................

I have never used ventilation. There are enough cracks for

ventilation. The temperature has never been a problem for me, and

in fact I have a couple of 75 watt incandescents also to keep it

warm in the winter. I use only one 30-watt germicical lamp. (The

others burnt out and I never replaced them; it works fine.)

UV dries varnish, but how? I think it is the ozone in small

quantities that is the reactive component. Disallowing ventilation

might actually help dry the varnish.

................................................................................

................................................................................

.

.........................

Hi John

Is that 30 watt germicidal lamp you use UVC?????.....Just in case

it is (as you'll probably be aware) I'll just add for anyone who is

not aware...... that UVC is very dangerous and any exposure to eyes

or shin is to be

" text-decoration: underline;">strictly

avoided.
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I have a PC fan on a switch at the top of my cabinet. I'll let the cabinet warm up and only switch on the fan if it gets too warm. You could put the fan on a thermostat which will switch it on automatically, but personally, I don't think that's necessary.

Photons catalyze a polymerization reaction in oil varnish. Both UV as well as infrared lights are involved in the reaction. Sunlight has plenty of UV.

My workshop is humidity controlled so any air entering is already humidified, therefore I don't feel I need more moisture in the cabinet. But it's a good idea to keep track of humidity levels since sudden severe dryness can cause seams to open or even cracks to form.

Oded Kishony

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Bob A,

There is more than one mechanism going on. One is the rearrangement of double bonds. Another actually incorporates oxygen, one atom at a time. This atom has to be split off from O2.

Strad had that little cupalo on top of his house. And they could have hung their violins outside. Strad even wrote a letter apologizing for the lateness of a violin because it needed "the hot rays of the sun." Ozone would be created in small quantities near the surface of the varnish. In any case, you need atomic oxygen. It seems a no-brainer. (And this is the catalysis you speak of)

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quote:


Originally posted by:
Melving
quote:


I'll just add for anyone who is

not aware...... that UVC is very dangerous and any exposure to eyes

or shin is to be avoided.

Thanks for your concern. I have a switch on the outside and turn it off when I open the door. I can put my hand in for a few seconds and the UV makes an odd odor with my skin oils. The bulbs are expensive, but you don't need a lot. When I started doing this, I think it was the only UV source I could find. (about 1960) The harm to the eyes is like what welders get.

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Melving is on target. The UV photons cure the oil. Ozone could also help, but it is the action of energetic photons that gets the oil to harden.

The muffin fan from eBay is about 3.5 inches in diameter. Below it are two one foot square filters stacked on each other. You need to make the filter much larger in area than the fan's area to reduce the air friction draw down. I bought the filter material at Home Depot, a hardware store. I like circulating air to remove any other distilates that could accumulate in the box. I can also dry spirit varnishes as well which indeed need circulating air.

My UV lamps do not make ozone. They are a combination of white tubes and clear tubes. Do not bother with the black tubes; they have low UV outputs. The violin wood really glows greenish and gets a nice suntan in a couple of days. Sometimes the wood gives off a wonderful smell. The lamps can be individually turned off leaving just the fan and/or rotation motor running.

Yes, glass will block out the UV. Make sure you stack glass plates to make the window about a half-inch thick, but one-quarter inch should suffice. I use half-inch plexiglass because I can saw it and drill mounting holes in it. (Try that with glass.) The window is a nice finishing touch for showing customers a violin in progress. Many people have remarked that they truly liked the window. Put a window in even if you already have a box. You will thank me later.

The rotation motor was from one of those dance ballroom rotating mirrored spheres. Check out eBay. The rotation motor ensures even exposure on all sides of the hanging violin. The box is lined with aluminum foil glued to the walls with a spray-on adhesive. The rotator also adds a nice effect for customers who can examine the spinning violin glowing in all its glory. You may not need it, but you may want it because it is "way cool".

I know some of my added features seem trivial, but they do add great appeal.

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Guys, be careful!

Even if humidity is 40% in your shop, if your light box is 100 degrees, humidity inside will be 15%.

At 120 degrees, that number drops to 8%.

This is dangerously low!

Even if the fiddle doesn't crack or come open, you'd need to let it sit for a long time after varnishing to reach moisture equilibrium before attaching the fingerboard, or fingerboard height will change.

The higher the temperature, the lower the relative humidity, so if you can keep the temperature down, you're part of the way to a solution.

Mounting the ballasts outside of the box will help, but the bulbs alone still give off a lot of heat.

If you make the box out of aluminum sheet with a wood frame, instead of making it with plywood covered with aluminum foil, it will transfer more heat to the outside air.

Still, I think it's good to use a fan with a filter (I use a large air filter from a truck, they're cheap and flow well). Mine has a high temperature cutoff switch in case the fan fails. This not only protects against low humidity if the temperature gets too high, but if the temperature gets much higher than planned, my varnish will run!

Even then, you need to add moisture. I do this by having the fan circulate the air from the box through flexible ducting to a container filled with water, and then back to the box. It's a closed system except for leakage, and a small adjustable vent that allows precise humidity control.

Also be careful about using germicidal or clear bulbs. In my experiments, they dried more slowly than BL or BLB bulbs, even slower than regular Cool Whites (which will leak enough UV to dry varnish).

A radiation guru told me that UVC or germicidal lamps will create polymer bonds, but that at the same time the radiation is energetic enough to break down polymer bonds. They're expensive, dangerous, and will dry and undry or degrade your varnish at the same time.

Drying time will also vary with varnish formulation and thickness. I've used varnish in the past that would dry overnight even with a thick coating. The varnish I use now will dry to the touch in a day, but takes a week before it's completely stable and ready for another coat.

A little about how flourescent bulbs work:

They all generate UV light. In a normal bulb, the UV causes a coating on the inside of the bulb to flouresce, and it's this glowing coating that gives off most of the visible light we see. On a BL bulb, the coating is formulated to give of a different wavelength which is stronger in the UV range. On both, the glass on the outside of the bulb is semi-opaque to UV and stops most of the harmful radiation beyond the near-visible range.

On a germicidal bulb, there is usually no coating on the inside, and the glass is formulated differently (quartz glass, I believe) to be transparent to most of the nasty wavelengths that are normally generated inside a mercury vapor bulb.

Bob A;

I don't think there's really anything un-natural about using UV bulbs to dry oil varnish. It's a way of using the active ingredients of natural sunlight, but allows us to dry varnish without having to pick out bugs and wipe off the bird doo-doo.

Ive done tons of experiments between drying with bulb-generated UV at many wavelengths and sunlight, and can't tell any difference, with the exception of germicidal lamps.

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quote:


(............)

Yes, glass will block out the UV. Make sure you stack glass plates to make the window about a half-inch thick, but one-quarter inch should suffice.
I use half-inch plexiglass because I can saw it and drill mounting holes in it. (Try that with glass.)
The window is a nice finishing touch for showing customers a violin in progress. Many people have remarked that they truly liked the window. Put a window in even if you already have a box. You will thank me later.(...................)


I do understand why you use plexiglass (easy to drill and bend) but this material (poly methylmetacrylate) does NOT block the UV radiation so be carefull for your eyes.

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quote:


Originally posted by:
DutchViolins

quote:


(............)

Yes, glass will block out the UV. Make sure you stack glass plates to make the window about a half-inch thick, but one-quarter inch should suffice.
I use half-inch plexiglass because I can saw it and drill mounting holes in it. (Try that with glass.)
The window is a nice finishing touch for showing customers a violin in progress. Many people have remarked that they truly liked the window. Put a window in even if you already have a box. You will thank me later.(...................)


I do understand why you use plexiglass (easy to drill and bend) but this material (poly methylmetacrylate) does NOT block the UV radiation so be carefull for your eyes.

I use OPTIX acrylic by Plaskolite.

Here is what they say about UV:

----

ULTRA VIOLET FILTERING

OPTIX acrylic sheet fi lters out between 80-90%

of the UV light within the damaging wavelength

area of 250-400 nanometers (See Fig. 1).

----

You can see Figure 1 in this document:

">http://www.plaskolite.com/fabguide/fabguide.pdf

As you can see it is superior to glass in filtering UV.

I also have UV measuring equipment and found nothing important coming through the window.

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There's some good info going on here!. I like Mike Molnar's

 ideas & also it was very interesting to hear what David

Burges does...from my experience it sounds

(unsuprisingly) like David has the drying cabinet/ light

box totally worked out!

My drying cabinet/light box (or whatever is the best name for it!)

is a bit different to what has been described so far. I buy cheap

second hand cello size sunbeds & trash them when they start to

fail. Presently I am using a rather nice 'Scandinavian style'

pine 20 tube double deck fast tan bed bought locally off Ebay for

substantially less than the cost of the UV tubes within. I use this

in my barn but even in a damp UK autumn with free air circulation

there is a very dry local climate in the range of the sunbed....All

I can say is that we should never under estimate the humidity

changes caused by light boxes & the effect on the instruments

within......There are many things to monitor!....Not much point in

doing a fine set up on a dried out instrument that will 2 months

later be totally rehydrated  have a 2 mm lower neck elevation

& probably sound a bit different too!....

On a lighter note & just to bait David Burges...What's with the

lorry air filter!....why not an ITG or K&N?

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Heh heh, Melving, you probably already know that while these filters flow better, they don't filter as well as pleated paper regardless of the manufacturers claims. They might be perfect for a "racing" drying box though

More importantly, the filters I use are huge for good flow even with a small fan, cost about 4 dollars, and there's no danger of oil residue getting anyplace as there might be with the oiled gauze filters.

I still get crap in my varnish though, even though I blow down my varnish room, myself, the fiddles and my varnish brushes with compressed air, have a filtered air supply for the room, and wear a shower cap. This junk must be sneaking over from a parallel universe............

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I remember reading a varnish scientific research (Dortmond, Germany, by Meyers, and others, I think) pointing out to the existence of dust in varnishes by old makers, and I believe they are right...

Imagine old Cremona, dirty roads (no pave streets) with horses raising dust all the time... I remember reading in an old English book how to fake fly craps, because old furniture has plenty of them, and modern life is a a bit "flyless".

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