Wood Stabiliser

[Screwdriver]

Anyone know what the difference is between vacuum and pressure when it comes to impregnating wood with <whatever> hardener?

I assume either vacuum or pressure will achieve the same ends...

 

[Agroshield]

I think vacuum is the only option.

You are never going to push anything into the pores of the wood no matter how high you pump up the volume. All it will do is compress the bubble of air that is in the material, which will then expand again when the pressure is released. In a vacuum, the air is removed and the returning atmospheric pressure pushes the preservative into what is now an empty void. Once the wood is impregnated, a positive pressure can help it stay there until it has set.

It is one of the many strange uses of English when you see 'pressure treated timber' written. It would be more accurate to call it vacuum treated timber. See this one: https://www.treatedwood.com/learn/how-wood-is-treated ("large steel cylinder; called a vacuum pressure vessel")

 

[Screwdriver]

I think vacuum is the only option.

You are never going to push anything into the pores of the wood no matter how high you pump up the volume. All it will do is compress the bubble of air that is in the material, which will then expand again when the pressure is released. In a vacuum, the air is removed and the returning atmospheric pressure pushes the preservative into what is now an empty void. Once the wood is impregnated, a positive pressure can help it stay there until it has set.

It is one of the many strange uses of English when you see 'pressure treated timber' written. It would be more accurate to call it vacuum treated timber. See this one: https://www.treatedwood.com/learn/how-wood-is-treated ("large steel cylinder; called a vacuum pressure vessel")

I don't think that's right. The mechanism for "removing" trapped air or filling "bubbles" with a preservative/hardener should be similar for both pressure or vacuum. A quick Google tells me "pressure treated" lumber is as you say actually vacuum treated but then in the same process also pressurised to make sure the voids in the wood grain are filled up (that's my laymans description of it!).

While it might seem counterintuitive since "hey how do you expect the bubbles to get out", you have to remember at a certain pressure, a bubble cannot exist submerged in liquid. It will simply disappear into the liquid. Remembering all you want is for the stabilising resin to be able to invade the wood grain. I got lost in Google trying to find out the exact process (look up "bubble point pressure" lol) and decided it would be much easier just to try it or find out from someone who's done it!

Anyhow, I am not sure what the practical differences are regarding pressure vs. vacuum, just that (a) pressure would be "easier" on the grounds that I have a compressor (!) and (b) I was just thinking about @DAPPH and his "pressure cooker".

 

[julianf]

Imagine the air is in a balloon.

If you put it in a vacuum, the balloon gets bigger, eventually pops, and the air is gone.

If you put it under pressure, the air gets smaller and smaller. But it's still there.

Take it out of the vacuum again and it's just the same as it was before.

This is what will happen to the air in the wood. You don't remove the air by pressure. You just make it smaller, whilst the pressure is applied. Take it back to atmospheric and it expands again to displace your stabilising solution in that area.

 

[DAPPH]

Anyone know what the difference is between vacuum and pressure when it comes to impregnating wood with <whatever> hardener?

I assume either vacuum or pressure will achieve the same ends...

One is a suck job the other is a blow job .

 

[Screwdriver]

Imagine the air is in a balloon.

If you put it in a vacuum, the balloon gets bigger, eventually pops, and the air is gone.

If you put it under pressure, the air gets smaller and smaller. But it's still there.

Take it out of the vacuum again and it's just the same as it was before.

This is what will happen to the air in the wood. You don't remove the air by pressure. You just make it smaller, whilst the pressure is applied. Take it back to atmospheric and it expands again to displace your stabilising solution in that area.

I am familiar with the science thanks. You didn't look up bubble point pressure did you?...

Anyhow, I am not going to pretend I know the right answer to this innocuous looking question but I do know it is fantastically more complicated than one might think.

Your balloon popping analogy is flawed. A balloon or meniscus bubble is different from a submerged bubble. If the "balloon" pops, are you suggesting the air just disappears? How does a bubble pop under water??

So anyhow, having said "I don't know", I thought I'd see if I could find out...

First observation: using a vacuum, the biggest pressure differential you can possibly achieve is 14.7 psi. That is, the difference between any trapped air or potential bubbles inside the wood and the evacuated "zero psi" vacuum you would struggle even to achieve in deep space.

Using pressure you can introduce a differential of, well, up to the pressure rating of your vessel. 45 - 60 psi easily (i.e. 3-4 times as much), 90 - 120 psi if you like living dangerously (or dying violently).

I **think** the pressure differential between the entrained or entrapped air and the stabilising liquid is the biggest factor in getting anything to happen. An hour of idle Googling tells me there are as many opinions as there are internet users. Some say using a vacuum "sucks out the air" as if displacing air from the vessel is the same as displacing it from whatever voids/capillaries it happens to find itself within the wood. Those commentators make a HUGE assumption that you can just pull all the air out until there's none left inside the wood but they forget, the wood is submerged in a viscous liquid; the air is trapped...

There has to be some mechanism for the air to lit. escape. I imagine (I don't know for sure) that using a vacuum is nominally "better" because it allows bubbles to grow and therefore "find their way out" from within the wood and escape to the surface. The bubble will be at say 14.7 psi, trapped inside a liquid at vacuum pressure and therefore have vastly increases bouyancy, escaping via gravity as the weighty liquid displaces it. Also, the low pressure affects any air/gas which had been dissolved in the resin allowing that to escape from the resin too. So far so good.

With a pressure pot, the opposite occurs. Yes entrapped air becomes a bubble inside the fluid bath and that bubble of air is pressurised now to say 45 psi. It will squash the bubble size a bit (not a lot because the physics tells us the linear relationship is to volume, not the "size" or diameter of said bubble). So three times more of a differential than you can achieve with a perfect vacuum, the same buoyancy since the liquid is incompressible but smaller in size = easier "escape"?? Also, being under pressure, more air can simply dissolve into the fluid. Henry's law: at 40-60 psi 5-10 times as much air can dissolve in water for example. (I think that's right, happy to be fact checked on that!). Once the liquid has permeated the displaced air, any air that would like to get back into the wood fibres when the pressure is released would have a difficult job pushing resin out of the way (I **think**)

If the air can dissolve into the fluid, it does not have to reform the same bubble in the same place once pressure is released. In fact, I rather suspect it will be vastly more likely to escape from the surface if pressure is slowly released or form bubbles at nucleation sites like Champagne in a glass. My guess is this might allow a lot more teeny tiny (invisible?) bubbles to form, rather than the larger voids we would like to remove.

Anyhow, you got this far in my little ramble, congratulations. This is as far as I got too...

If anyone would like to pick this up and run with it, be my guest. The situation is either vastly more complicated than at first glance or I am vastly more stupid than I had hoped...

 

[julianf]

I am familiar with the science thanks. You didn't look up bubble point pressure did you?...

Anyhow, I am not going to pretend I know the right answer to this innocuous looking question but I do know it is fantastically more complicated than one might think.

Your balloon popping analogy is flawed. A balloon or meniscus bubble is different from a submerged bubble. If the "balloon" pops, are you suggesting the air just disappears? How does a bubble pop under water??

So anyhow, having said "I don't know", I thought I'd see if I could find out...

First observation: using a vacuum, the biggest pressure differential you can possibly achieve is 14.7 psi. That is, the difference between any trapped air or potential bubbles inside the wood and the evacuated "zero psi" vacuum you would struggle even to achieve in deep space.

Using pressure you can introduce a differential of, well, up to the pressure rating of your vessel. 45 - 60 psi easily (i.e. 3-4 times as much), 90 - 120 psi if you like living dangerously (or dying violently).

I **think** the pressure differential between the entrained or entrapped air and the stabilising liquid is the biggest factor in getting anything to happen. An hour of idle Googling tells me there are as many opinions as there are internet users. Some say using a vacuum "sucks out the air" as if displacing air from the vessel is the same as displacing it from whatever voids/capillaries it happens to find itself within the wood. Those commentators make a HUGE assumption that you can just pull all the air out until there's none left inside the wood but they forget, the wood is submerged in a viscous liquid; the air is trapped...

There has to be some mechanism for the air to lit. escape. I imagine (I don't know for sure) that using a vacuum is nominally "better" because it allows bubbles to grow and therefore "find their way out" from within the wood and escape to the surface. The bubble will be at say 14.7 psi, trapped inside a liquid at vacuum pressure and therefore have vastly increases bouyancy, escaping via gravity as the weighty liquid displaces it. Also, the low pressure affects any air/gas which had been dissolved in the resin allowing that to escape from the resin too. So far so good.

With a pressure pot, the opposite occurs. Yes entrapped air becomes a bubble inside the fluid bath and that bubble of air is pressurised now to say 45 psi. It will squash the bubble size a bit (not a lot because the physics tells us the linear relationship is to volume, not the "size" or diameter of said bubble). So three times more of a differential than you can achieve with a perfect vacuum, the same buoyancy since the liquid is incompressible but smaller in size = easier "escape"?? Also, being under pressure, more air can simply dissolve into the fluid. Henry's law: at 40-60 psi 5-10 times as much air can dissolve in water for example. (I think that's right, happy to be fact checked on that!). Once the liquid has permeated the displaced air, any air that would like to get back into the wood fibres when the pressure is released would have a difficult job pushing resin out of the way (I **think**)

If the air can dissolve into the fluid, it does not have to reform the same bubble in the same place once pressure is released. In fact, I rather suspect it will be vastly more likely to escape from the surface if pressure is slowly released or form bubbles at nucleation sites like Champagne in a glass. My guess is this might allow a lot more teeny tiny (invisible?) bubbles to form, rather than the larger voids we would like to remove.

Anyhow, you got this far in my little ramble, congratulations. This is as far as I got too...

If anyone would like to pick this up and run with it, be my guest. The situation is either vastly more complicated than at first glance or I am vastly more stupid than I had hoped...

I was trying to help you out, not have a row.

 

[Screwdriver]

I was trying to help you out, not have a row.

I'm just trying to work it out. It is surprising how much variability there is in the "information" that's out there.

I strongly suspect vacuum stabilisation is "better", just trying to find out why and perhaps how much better considering the easy availability of compressed air (in my case).

 

[timgunn1962]

My (limited) understanding: As an overly-simplistic model, dry wood can be considered as a lot of fine tubes in a bundle. If you start off with the tubes full of air and immerse the bundle horizontally in a viscous liquid, the liquid will start to enter each end of the tubes. If you immerse to a greater depth or otherwise increase the liquid pressure, the liquid will move further along the tubes, until the pressure of the bubble of air trapped in the tube matches the liquid pressure. The tube constrains the bubble, so we can consider in terms of length only.


Again, massively oversimplifying, if you start with atmospheric pressure (1 bar absolute) and 7" long tubes, then pressurize to 90 psi, 6 bar (gauge), 105 psi, 7 bar (absolute) the bubble will reach 7 bar(a) when it is 1" long. You'll get 6/7ths saturation.

The saturation will depend on the ratio between the low pressure and the high pressure.

If you apply a "hard" vacuum before immersing, there is no air trapped in the tube, so when pressurized, the liquid can completely fill the tube.

In reality, you won't pull a perfect vacuum, but even a cheap and readily-available vacuum pump should get you under 1 mbar (a) to give 99.9% saturation when back to atmospheric pressure.


I gather the "best" of the companies that specialize in stabilizing wood use a combination of the two: apply vacuum to the wood, then pressurize to obtain maximum penetration. I imagine this is more about time and cost: getting the vacuum down by a factor of ten will, at some point, become more difficult/expensive than whacking the pressure up to ten bar.

 

[Agroshield]

...considering the easy availability of compressed air (in my case).

You can buy devices that you put a lot of compressed air into one end and that produce a vacuum at the other end, e.g. https://uk.rs-online.com/web/p/vacuum-generators/2022429 Maybe they offer some potential.

 

[timgunn1962]

The compressed-air vacuum generators are OK for suction cups,etc for handling, but the one in the link shows maximum vacuum of 89%. I assume that'll be about 110 mbar(a).

I think stabilizing needs a "proper" vacuum pump to work effectively.

 

[Screwdriver]

I have seen suggestions for using some sort of remote control mechanism inside the chamber for lowering wood into the “dip” after achieving a decent vacuum. That does make sense but slightly overkill for home grown stabilising (I might get the chance to try this!).

 

[DAPPH]

I have seen suggestions for using some sort of remote control mechanism inside the chamber for lowering wood into the “dip” after achieving a decent vacuum. That does make sense but slightly overkill for home grown stabilising (I might get the chance to try this!).

You seem to have information overload and are over thinking it , my heads a bit fluffy at present due to medical drug probs so bear with me ..


Big timber preserving tanks use a similar principle to this and use push & vacuum pressures for deep penetration of the fluids

I understand that for simple home shop cases ;-

Just lay the wood in a sealable cylinder with a visible heavy glass jar overflow reservoir on the evacuating end & the same on the other end to hold the stabilizing fluid . Do a few simple calculations to get the volume of the cylinder and ad a bit more then make the reservoirs to suit ,

Seal it with stop taps , open one to develop the supplied vacuum & close off , then open the other tap to slowly pull in the stabilizer fluid, letting the vacuum draw the fluid in,
As it slows down restart the vacuum till you see fluid enter the outlet reservoir then stop the vacuum ,/ close it ff for a few minutes to let the drawn in fluid impregnate what you want to stabilize , drain off then put the wood somewhere sensible to cure ,

Look up a few YouTube's on stabilizing wood for gun butts & stocks etc . there used to be some reasonable ones.

Try putting , " How is stabilized wood made ? " into your search engine.

Give it a go & tell us what the result is .... Eddison is supposed to have done 1200 experiments before he & his team got a successful light bulb that burnt for more than 10hrs .

 

[Screwdriver]

Thanks yes seen all that, plus all the warnings about how pulling a vacuum on wood can destroy your vacuum! Lots of acids and nasties get all over your pump etc. All seems a bit of a faff and I was really only trying to get a handle on the practical (empirical) difference between vacuum and pressure treatment.

Wasn’t expecting the Spanish inquisition.

 

[Agroshield]

Wasn’t expecting the Spanish inquisition.

There is some confusion here about who is tied to the rack and who is turning the windlass.

 

[Robotstar5]

Wasn’t expecting the Spanish inquisition.

 

[Screwdriver]

There is some confusion here about who is tied to the rack and who is turning the windlass.

Other threads are available...

 

[Screwdriver]

Million thanks to @timgunn1962 who popped over - with a vacuum pump! An amazing device, vastly more capable than I could hope for which can be pressed into service for a multitude of purposes. During the (compulsory) shed tour, we discovered a nice looking vacuum chamber which I have been meaning to refurbish for some time but looks ideal and is rated for a 15psi vacuum. I hope that is grammatically correct but you know what I mean.

Anyhow, I will also be able to use this spectacular machine (specs include 2.94 CFM, 5.00 m3/hr !) for vacuum forming plastics, maybe as part of a vacuum clamp any any number of other uses which I'll think of later...

Interestingly, it might also help dry out my wood blanks, just by placing them inside a vacuum chamber, whether that's part of a stabilising process or not. One assumes having a chunk of wood in a vacuum will help reduce the moisture content in the short term, perhaps at the risk of extreme warping?? Anyhow, I will keep this thread alive and start getting the old vacuum chamber ready. It's only a cosmetic restoration, physically looks pretty solid.

Brilliant to meet you @timgunn1962 and fascinating to talk with someone with such a deep technical knowledge in so many of our shared interests. You're a proper geek!

 

[DAPPH]

Just had a thought about drying the blanks .. do a few throw awayable experiments first .
Quickly drawing out the water under vacuum might leave the fibres of the wood looking like a loofa instead of them slowly shrinking to a more dense material .

Was thinking about the pressure treated fence posts done under vacuum and pressure, the fibres are soft and open as comared to a well slow seasoned post cut by yourself .

 

[DAPPH]

Million thanks to @timgunn1962 who popped over - with a vacuum pump! An amazing device, vastly more capable than I could hope for which can be pressed into service for a multitude of purposes. During the (compulsory) shed tour, we discovered a nice looking vacuum chamber which I have been meaning to refurbish for some time but looks ideal and is rated for a 15psi vacuum. I hope that is grammatically correct but you know what I mean.

Anyhow, I will also be able to use this spectacular machine (specs include 2.94 CFM, 5.00 m3/hr !) for vacuum forming plastics, maybe as part of a vacuum clamp any any number of other uses which I'll think of later...

Interestingly, it might also help dry out my wood blanks, just by placing them inside a vacuum chamber, whether that's part of a stabilising process or not. One assumes having a chunk of wood in a vacuum will help reduce the moisture content in the short term, perhaps at the risk of extreme warping?? Anyhow, I will keep this thread alive and start getting the old vacuum chamber ready. It's only a cosmetic restoration, physically looks pretty solid.

Brilliant to meet you @timgunn1962 and fascinating to talk with someone with such a deep technical knowledge in so many of our shared interests. You're a proper geek!

Can you post the model number etc of the pump or photo the data plate please ?