Shellac Hardness compared to other finishes

[Tetsuaiga]

I'm trying to find out how hard shellac is compared to other finishes but don't really be able to find much information.

Is anyone able to give me a comparison with something common like Polyurethane, i'm interested purely in hardness not durability to moisture, water, heat.

I've also read that button shellac is harder than other forms because it's heated which causes extra polymerisation.

Thanks

 

[mouppe]

It's pretty hard but it goes on thin so I think that has to be taken into account.


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[Tetsuaiga]

Yes it usually does built up very slowly. You can build it thicker but it is brittle so i think the thicker tbe more chance there is of developing cracks or crazinI imagine brittleness and hardness go together, i dont know if you can get fleible yet hard wood finishes =s.

 

[Matt@]

be aware that this sort of finish is often as hard as the wood its on thus pine will dent easily and dense oak will almost be rock hard.....

 

[thick_mike]

be aware that this sort of finish is often as hard as the wood its on thus pine will dent easily and dense oak will almost be rock hard.....

Exactly right. I used to design paint systems for plastic components in the automotive industry. We invariably found that the hardness of the finish (notch hardness or pencil hardness) was directly proportional to the hardness of the substrate. So any finish will be as hard as the wood as long as the finish is put on thinly enough. Thicker hard finishes on softer wood will be prone to cracking.

You might get some improvement in surface hardness if you have a finish that penetrates the wood and soaks in and reinforces the fibres, like an oil finish, but I don't have any experience of those.

 

[MIGNAL]

I've spent countless hours trying to work this out.
As others have stated it's difficult to judge hardness when a finish has been applied to wood. Most of the time you are simply compressing the underlying wood. There's also factors such as drying time. Actually that should read hardening time. That can be any number of weeks, months or even years. Different finishes will take longer than others to reach a point where they aren't really going to get much harder but that can take a mighty long time, even with evaporative finishes like Shellac. Then there is the factor of 'hardness' and 'toughness', which aren't quite the same thing.
In short I can't answer your question but I can suggest that you take a flake or a button and try scratching it with a fingernail (or other implements). I think you should find that perhaps it is a little harder than you might imagine. Nitrocellulose does have a reputation for being harder than shellac but it also has a reputation for being prone to cracking. I don't know much about Polyurethanes, natural oil/resins are more my thing.

 

[AJB Temple]

I am no expert....I have been using shellac and various shellac mixes on musical instruments for years, and have had a fair bit to do with piano refinishing. Shellac is much less durable than Urushi lacquer (Chinese lacquer) but much easier to apply to a good finish. There are now shellacs mixed with other plasticisers that are used on stringed instruments that improve durability. Piano refinishers generally reckon that polymer finishes are far more durable than shellac, but a lot harder to repair. Poly finishes can be made very hard indeed, to the point of being brittle, and some musical instrument makers have experimented with this. I am most familiar with it on pianos but as I said, I am not an expert. You could find out more by talking to one of the piano restoring experts, such as Richard Dain at Hurstwood Farm. I can't remember now who he uses for major refinishing jobs but he handles a lot of fine pianos with both traditional and poly finishes. They are often looking for extremely high gloss and durability (chip and scratch resistance).

 

[thick_mike]

Traditional way to test hardness of a coating is to draw a pencil over it (pressing hard enough to crumble the lead). Work up through the pencils from 6B to 6H. The first pencil that leaves a permanent mark on the coating is taken to be the pencil hardness of that coating. Obviously the pencils should all be sharpened to the sam degree before you start the test.

Toughness can be done in a similar way, but looking for tearing of the film rather than an indentation.

 

[Tetsuaiga]

Thank you for all the replies.

I think the point about the underlying wood making a difference is probably something I haven't appreciated as much as I should. Although I do wonder if you can overcome that by just making a very thick finish?

The pencil test is something i'd like to do though i'm a little confused, do you press on with the pencil enough to leave the graphite mark or is it enough force to make a physical indentation? I guess the second makes more sense as then you find out which material dominates the other when forced together, maybe as I don't have any real experience I don't understand how soft some pencils are.

I have looked into some of the oriental lacquers and they sometimes use ground deer horn mixed with lacquer to create a base over soft woods. Then apply more lacquer over that, it's an interesting idea.

 

[thick_mike]

Thank you for all the replies.

The pencil test is something i'd like to do though i'm a little confused, do you press on with the pencil enough to leave the graphite mark or is it enough force to make a physical indentation? I guess the second makes more sense as then you find out which material dominates the other when forced together, maybe as I don't have any real experience I don't understand how soft some pencils are.

Just press hard enough to see some pencil lead crumble. A 6B pencil is pretty soft.

 

[custard]

You might get some improvement in surface hardness if you have a finish that penetrates the wood and soaks in and reinforces the fibres, like an oil finish, but I don't have any experience of those.

This idea that oil finishes soak into the fibres and strengthen the wood seems to me like a load of marketing horse droppings.

Firstly oil barely soaks in to any meaningful depth. Even the most thorough application of oil can be easily removed with a few strokes of 80 grit abrasive paper, so it's difficult to see how it can have penetrated more than about 0.1mm.

Secondly, pour a thin film of oil onto a sheet of glass and let it dry for a few days. You can scratch it with your fingernail. So I'm sceptical that it is bestowing any hardening advantage at all.

I know that a US finishing oil manufacturer ran advertising for many years making this nonsensical claim about deep penetration and wood hardening, and somehow it's an idea that's been widely adopted. But it's just snake oil salesmanship that doesn't stand up to much scrutiny.

 

[Sgian Dubh]

This idea that oil finishes soak into the fibres and strengthen the wood seems to me like a load of marketing horse droppings.

I think you're correct. Wood finishes of all types are predominantly a surface coating. Their penetrative powers are minimal. True, they do penetrate the upper microns of the wood's surface in order to create a bond, but beyond that there is little or no penetration. You only have to look at the kind of penetration obtainable through surface 'penetrating' treatments such as common furniture beetle fluids to recognise (white spirits being a common and runny carrying medium) that deep penetration of a more viscous finishing product to recognise that such claims are nonsense.

The one debatable exception I can think of is standing, end grain down, a length of wood in a container of liquid finish such as linseed oil, where the tubular vascular tissue can draw the fluid up into the wood to some extent through capillary action. I believe that wooden planes were sometimes subjected to this kind of treatment, although I could be wrong on that score, and I'm not at all sure just how much finish penetration actually occurs, and even if it does, most of the penetration will be within the open vessels rather than within wood cell lumen, with their barrier of the cell wall. Slainte.

 

[kfenelon]

Shellac is more of a decorative product and I would be more concerned that it has very little heat resistance and should not be used on tables that could have hot cups placed on them. Shellac is applied in very thin coats and if French polished can give fantastic results. The French polish you can purchase from DIY stores contain a lot more than just shellac and methylated spirit to aid drying
Ken

 

[ED65]

Is anyone able to give me a comparison with something common like Polyurethane, i'm interested purely in hardness not durability to moisture, water, heat.

Bit of a late addition to your thread but there's a table given on the following page that might be just what you were looking for. It's related to the pencil test mentioned already in the thread: http://pencilpages.com/articles/simmons.htm

I don't know how definitive this is WRT shellac though, which I had previously thought was quite a hard finish!

The author mentions that the hardness of any generic coating can vary, as is easily seen by the variations in different types of polyurethane. And with shellac, aged shellac is known to be less hard than fresh shellac (even to the point that you may be able to leave fingerprints in it after it has fully dried) so that's obviously one way it can vary, but it is a natural product to begin with and those are rarely consistent, I'd expect there to be some difference from batch to batch or between the various colours. And on top of that there are waxed or dewaxed shellacs.

 

[Tetsuaiga]

Thank you ED35, I was a bit surprised how low shellac came down that list, I wonder what sort of level of build it was. I suppose to be a meaningful test they should all be equal in thickness.

 

[MIGNAL]

Hmmm. . . not entirely sure about that test. Firstly he gives shellac only one week to dry/harden. I suggest that's a little too early to be testing any finish. One month would be more like it, preferably 3 months.
Secondly he does not indicate which type of wood he is using. From experience it's pretty difficult to test finish hardness unless the wood you are using is seriously hard. You tend to compress the wood that is underneath the finish and it's all too easy to get a false reading. I use Ebony. Of course you could always try the pencil test on an actual shellac flake/button.

 

[ED65]

Agreed, definitely not definitive, but the author stresses that generic results won't apply to individual versions of whatever finish. I think the whole point of it is that this is something that readers should repeat themselves.

Secondly he does not indicate which type of wood he is using. From experience it's pretty difficult to test finish hardness unless the wood you are using is seriously hard. You tend to compress the wood that is underneath the finish and it's all too easy to get a false reading. I use Ebony.

I see what you're saying here, but if you do the test on a sample of the wood that you'll be using for a project don't you get a direct reading of how it'll stand up on that wood? I think this is actually more important than a table of absolute hardness, for example on most common softwoods some give might well be desirable and you don't want a particularly hard finish.

Of course you could always try the pencil test on an actual shellac flake/button.

I thought of that but I wasn't sure if those somehow don't represent how the finish is when solved and applied. It should be identical once the alcohol is fully gone and the shellac has coalesced, but I wasn't sure.

 

[MIGNAL]

You need to test the hardness of the material/finish without the factor of the wood density. At least that will tell you the comparative hardness of each finish, a constant. It's much easier and more reliable to do that test when the underlying material has little 'give'. A 'tough' finish isn't necessarily the same thing as a hard finish. Toughness is usually refered to as being able to take the knocks. Early forms of Nitrocellulose were hard but they tended to be very brittle and over time they often cracked, chipped and crazed. In one sense they were hard but not very tough. Of course it usually took a decade or so for the finish to exhibit these tendencies. Shellac is similar in that it can change nature over a long period of time, even becoming impervious to it's own solvent alcohol. It certainly gets harder over a period of years but that's also true of oil based finishes, which may take even longer.

 

[ED65]

You need to test the hardness of the material/finish without the factor of the wood density. At least that will tell you the comparative hardness of each finish, a constant.

I agree that's a good thing to know in absolute terms, and this test on a suitable very hard wood will give a reading on that. But I was referring to toughness over hardness in that the results of this testing won't directly inform how each of those finishes will stand up on woods that aren't as hard as the chosen test substrate, so e.g. a bad bruising blow on pine will crack a hard lacquer but won't crack poly.

 

[the bevel you know]

I'm a French polisher and furniture restorer (of 35 years experience) so can contribute a little info here if I may. As has been hinted, a shellac finish is only really as hard as the surface it is being applied to. The other factors to consider are drying time and depth of polish. Shellac, dries by evaporation, so if you apply a lot in one siting, expect to wait a week or two for it to get to maximum hardness. That's why we generally take time to build up to the desired finish, allowing a few days in between sessions for the last application to dry. And polish does sink a little as it dries so an acceptable finish can rarely if ever be attained in one go. I use the minimum amount of oil as this slows up drying as well. It can create as many problems as it solves if not removed properly.

As an aside. People often ask me how many coats do I apply to get the desired result? But you can't really count, or would never bother. It's a continuous process dependent on wood type, how hard/porous the wood is, and environmental conditions to some extent. Ambient heat and humidity can also affect how the polish goes on and dries. The number of coats is irrelevant.

I buy raw shellac buttons or flakes as I like to make up batches to my own specification. The buttons are extremely hard (can be up to 1/8" - 3mm thick) and it takes some effort to scratch them or snap them in half. But once they have been dissolved in methylated spirit I don't think it ever gets back to the hardness it has in raw button form. Or at least not for a very long time.

As a general rule, polishers try to use the minimum amount of polish to achieve the desired result. Less, is definitely more. Once the grain is filled there is little or no point adding more polish if it's a flat even surface you are after. Open grained timber might need grain filling to save on polish and time. Harder, densely grained timber, the polish tends to sit on the surface rather than penetrate which can make it prone to scratching. If I'm polishing an old piece of seasoned mahogany, you could pretty much regard it as hard as any of the modern varnishes that dry by chemical reaction or polymerization. You couldn't leave a dent from sustained thumbnail pressure for instance. It's as hard as it needs to be which is why it is still being used.

Shellac's only real flaws are low resistance to alcohol or heat. Other than that it's pretty durable. Hope my input is of some use.

P.S. Button, or French polish is the hardest shellac. Garnet next, but almost as hard. Has some additive to make it dark. Pale or transparent polishes tend to be soft as a result of the bleaching process involved to lighten them.