Furniture Education: How Wood Ages, Part II — Finish

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Today Dave is back with more on how furniture ages, and a little bit more on what you can do to achieve the look you want with finish selection.

Our last post talked about the ten­den­cy of wood to change as it ages, based on it’s inter­ac­tion with the atmos­phere: air and water.  Today we’re going to look at the biggest impact you can make on that change through con­trol of those two ele­ments in your home, and fin­ish selec­tion.

As we men­tioned last week: Wood is made from a vast array or organ­ic mol­e­cules. These mol­e­cules in wood react to heat, light, humid­i­ty, air, and all the things that touch the wood’s sur­face over the decades. A major pur­pose of fin­ish is to pro­vide a bar­ri­er to these things – it’s the “stain­less” equiv­a­lent for wood. The type of fin­ish and its sta­bil­i­ty in the face of those assaults change how quick­ly the wood reacts to its envi­ron­ment.

Let’s con­sid­er the envi­ron­ment first. Heat as a fac­tor by itself is only sig­nif­i­cant at extreme vari­a­tions, and oth­er­wise most­ly impacts humid­i­ty.

Humid­i­ty vari­a­tions make the wood swell or shrink, and this occurs nat­u­ral­ly as the sea­sons change the air from warm and humid to cool and dry. One of the biggest dif­fer­ences between high end fur­ni­ture and mass mar­ket fur­ni­ture is in care and under­stand­ing of wood as it han­dles humid­i­ty: prop­er­ly designed fur­ni­ture accounts for the fact that wood expands and con­tracts with humid­i­ty changes. So as long as your piece is built prop­er­ly, fur­ni­ture can han­dle changes in humid­i­ty, but cup­ping (curl­ing of flat boards across their width) is a fre­quent issue.  Humid­i­ty con­trol in your house is the best present that you can give to your fur­ni­ture: a humid­i­fi­er for the dry win­ter months, and a dehu­mid­i­fi­er or air con­di­tion­er if sum­mer humid­i­ty is high. My shop is humid­i­ty con­trolled at 40 – 50% year-round to ensure sta­bil­i­ty in my pieces as they’re being con­struct­ed.

There’s not much that can be done about air, oth­er than encas­ing the piece in thick plas­tic. That’s just ugly or means you live in a muse­um, so let’s not wor­ry about it.

Light we touched on briefly in our last post on this top­ic, but to reit­er­ate: light accel­er­ates the rate of change for wood tone because it con­tains UV rays. Because of the expo­nen­tial expo­sure to light that exte­ri­or woods face, exte­ri­or fin­ish­es often con­tain a UV inhibitor – a pho­tore­ac­tive mol­e­cule that pref­er­en­tial­ly absorbs the UV so that the wood under­neath doesn’t. That increas­es the longevi­ty of exte­ri­or wood but at the cost of a thick, very yel­low, sur­face coat­ing. That’s fine on your ipe deck, but not so much on a cher­ry desk with inlays.

For inte­ri­or fin­ished: some oils are also pho­tore­ac­tive – they dark­en with sun­light. Lin­seed oil is very pho­tore­ac­tive, boiled lin­seed oil less so. Tung oil is a lit­tle reac­tive also. Before 1960, var­nish­es were made with lin­seed oil, so guess what col­or they turn? That is one of the rea­sons old fur­ni­ture is so dark. It’s not actu­al­ly the wood, it’s in the fin­ish. Mahogany is red when fresh­ly worked. “Mahogany” col­ored mod­ern fur­ni­ture is stained to be the col­or of old var­nish on dark red wood, because that is now the col­or peo­ple asso­ciate with mahogany.

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So how does this inform my work?  Since I make most­ly dec­o­rat­ed fur­ni­ture, I want fin­ish­es that are very pale: with as lit­tle col­or shift as pos­si­ble. Also, I pre­fer to use fin­ish­es that do not dark­en with time. I don’t use boiled lin­seed oil for both rea­sons. Oil based var­nish is almost unavail­able any­ways these days due to air qual­i­ty reg­u­la­tions that pro­hib­it some of the key sol­vents.  That is a sad thing for cus­tom fur­ni­ture, but a good for soci­ety.

That leaves us with my pre­ferred tra­di­tion­al fin­ish: shel­lac.  You may have heard of shel­lac in the con­text of a nail salon, but it is an excel­lent fin­ish with many desir­able prop­er­ties. Shel­lac ages pret­ty well, and also can be bought “super-blonde” – a very pale yel­low. Unfor­tu­nate­ly, water repel­len­cy isn’t one of shellac’s fea­tures, so I use it alone only when pro­tec­tion from water is not a require­ment. When it is, I top­coat the super blonde shel­lac with water based polyurethane. I know that purists don’t approve. It looks a lit­tle plas­tic, because it is plas­tic. How­ev­er, a thin coat­ing pro­tects the wood from water while min­i­mal­ly build­ing thick­ness. The lat­est prod­ucts are also “water white”: they add no notice­able hue at all*. They are quite an amaz­ing improve­ment from their 10 year old for­bear­ers.

In addi­tion to how the wood itself changes col­or over time, it acquires addi­tion­al char­ac­ter called “pati­na.” Pati­na is the result of the dirt and oil and dust and such that gets on a piece of fur­ni­ture, then gets caught in the paste wax that is the tra­di­tion­al pro­tec­tant for fur­ni­ture in the house. Tra­di­tion­al paste wax is a pret­ty good means of pol­ish­ing your fur­ni­ture – much bet­ter than aerosol based fur­ni­ture pol­ish­es that have sil­i­cone in them so that they lev­el bet­ter.  Fin­ish­es, by the way, do not need to be fed. That is pure mar­ket­ing gib­ber­ish. Clean your fur­ni­ture when it needs it, and apply wax when it looks dull, or every twen­ty years, whichev­er comes first.

Prop­er care of your wood fur­ni­ture, and under­stand­ing fin­ish selec­tion from the start, will mean that your piece will mel­low and acquire char­ac­ter as it ages, ready to grace your family’s home for gen­er­a­tions.

* A note: in hard­ware stores you may notice fin­ish­es adver­tised as “clear.” It should be not­ed, that this is NOT the same as “colour­less.”

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William Brown - What prod­uct do you use for the water -based poly?



Jenn H - Hi Bill!  Thanks for your ques­tion — Dave uses Gen­er­al Fin­ish­es water based poly.  He uses the Enduro Sand­ing Seal­er, fol­lowed by the Enduro Clear Poly as a top­coat.  Both are spray appli­ca­tion only, but he has been real­ly pleased with the results — water white fin­ish with excel­lent lev­el­ing and quick dry time.  I hope this is help­ful!

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