Stabilizing Wood: Physics, Chemistry, Materials, Techniques, and Performance: "Just the facts Man"

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What gets in the way of filling the porespace with resin? – WATER

A funny thing happened on the way to the (knife) scales:

Several weeks ago I scored a bunch of basically dried black walnut from a local guy who sources it from southern MN farmland, and turns it in to slab tables (they are gorgeous!). He has a supply of “ends” which are just perfect for knife scales. Although these were “kiln dried” they had been sitting in a pretty uncontrolled garage. So … after cutting the slabs down to 2x2” blocks, some of my “leftovers” were ½” slabs (perfect for the small “dinner” knives I am making for my wife). I decided to see if I could “push” them to total dryness by putting them in to a food dehydrator I have (145F max temp, circulated) – I also wanted to see just how dry they had stayed. Here they are – just a few small pieces:
upload_2019-7-22_10-56-6.pngv


(I’ll explain the “138” in a little bit).

Before I put them into the dehydrator, I weighed them. They were 145g (according to my kitchen scale, above). It is not perfect, but probably accurate enough for current purposes. I DO have access to a laboratory scale accurate to 0.0001g – might pull it out later if seems necessary . Anyway, I put these babies into that food dehydrator (think of it as a small but very active “drying box” at 145F). After weighing once in a while as things went, they lost weight, and leveled out (i.e. lost no more weight) after 24 hours at 135g. (the "138" was measured after about 4 hours after taking them out of the dehydrator. they had pulled water out of the air and regained that weight all on their own)

Ok, they lost 10g of water. 10g/145g = ~7% moisture initially. Seems reasonable.

But wait – this is 7% by weight – and water is more dense than (dry) wood. So – what does this LOOK like?

10g of water is 10mL of water. So – this is what that volume of water looks like next to those slabs of wood:

upload_2019-7-22_10-56-36.png

That is a LOT of water relative to the volume of the wood it came out of. I do not know about you – but faced with the visual reality (not just some number like “6%”, I was rather taken aback. Where does all this water go in the wood?

Ordinarily with something like this, we speak about adsorption of water to the surface (or inner surfaces) of something (and yes, the word is “adsorption”, not “absorption”). But adsorption to something is a molecularly thin surface layer – not a lot. However, there is a phenomenon called “capillary condensation”, where liquid water condenses and fills very small spaces (capillaries), even at pretty high temperatures (the explanation for why this is comes from statistical thermodynamics – think lots of big hairy equations … so lets not go there here. Suffice it to say that this is an observed and measurable phenomenon). Also suffice it so say that there is a lot of journal data out there to confirm that capillary condensation is a very active mechanism in moisture retention in wood.

If you have a lot of capillaries, you can hold a LOT of water. The other aspect of capillary condensation is that it is very, very, highly biased towards putting that water into liquid form in the capillaries. Even with very low ambient humidity, the forces involved will “pull” the water out of the air and cause it to condense in those capillaries. This mechanism is, in fact, exactly how many very effective desiccants work. The other thing to know is that the amount of liquid water in the capillaries exists in an equilibrium with the ambient humidity, with the balance of that equilibrium being dependent on temperature (the lower the temperature the more capillary condensation, the higher the temperature the less the capillary condensation). This water will not dry out with time – in fact the wood will actively “pull” water into it, even with very low ambient humidities.

So – going back to our “model pore”, water in the porespace looks something like this:
upload_2019-7-22_10-57-3.png
Where the blue areas are meant to be condensed (liquid) water.

Three points:

1) when you pull a vacuum and pull the air out of the porespace, you will not remove very much of the water: the air will just kind of push the water aside to the walls where it will stay, and then when the air stops rushing by, the water will just “pool up” again in the small pores (remember – the forces creating these liquid water pools are extremely strong).

2) When you try to force acrylic resin into the porespace, you might succeed in “pushing” the water deeper into the pores, but you will not remove it. Ultimately, the volume taken up by this water is pore volume that the resin will not fill.

3) Remember there is a LOT of this liquid water, filling a lot of the porespace. Not good for filling as much of the porespace with resin as you can.

So just how do you go about getting rid of that liquid water in the wood? One way to look at this is from information in the Wood Handbook ( https://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf ) which gives some cool information on water content versus temperature and humidity. One figure they present on equilibrium moisture content is this:
upload_2019-7-22_10-58-0.png

So – you can reduce moisture content by varying either ambient temperature or ambient humidity. Accurately controlling ambient humidity, especially to very low values is HARD (I have done it while characterizing desiccants – it takes a $15-20k piece of equipment, and a lot of tuning of the controllers). So for our practical purposes, that leaves only TEMPERATURE. Lets pick on the 50% RH data (the handbook also gives numerical values). The relationship at 50% RH between equilibrium moisture content and temperature looks like this:
upload_2019-7-22_10-58-38.png

Look closely at that curve. Even if you heat the wood to 240F, you are only going to be able to remove about half of the residual moisture, no matter how long you heat it for.


Now – these are more or less average values. The actual values will depend on species – but this should be enough to make the points. And the points are:

1) If you do nothing to remove moisture in the wood before trying to impregnate with resin, there will be a LOT of liquid water in the pores, and that liquid water will block the resin from filling a lot of the porespace.

2) The only practical way of removing that water is to heat the wood – the hotter the better (without burning the wood of course). Even then, you will never get all of the water out.

3) Capillary condensation is a powerful and fast mechanism. Once you remove the wood from the oven (or dryer) it will immediately start pulling water out of the air. A lot of it. Quickly. So protect it while it cools (zip loc bags with air removed, or better yet vacuum sealed food storage bags. (do NOT put the wood into the resin while hot - the heat will start the resin curing before you can get it into the pores)

One other thing: moisture meters, especially the two-prong conductivity meters, become very inaccurate at low values of moisture (the reason comes from something called “percolation theory” – again coming from the petroleum industry – really cool (you can demonstrate it with M&M’s ) – but that is way beyond the current topic). So … if you want to track the moisture content of your drying wood as you heat it, get a digital scale and let weight loss over time guide you. Timeframes for typical scales or 2x2 blocks will likely (just based on my recent experience) be measured in days. K&G does bake their stuff prior to impregnating with resin, and they use a moisture meter to judge degree of dryness.
 
Moisture is one of the big killers of stabilized wood, and why myself, K&G and others use an expensive contact moisture meter.

When doing home stabilizing, a lot of people will not fully dry the wood. Honestly, I would say wood moisture is responsible for about 75% of peoples issues with wood, both natural and stabilized.

If you put wet wood into a stabilizing set up, the water will boil in the vacuum, and it will interfere with the curing of the resin, leaving a sticky, cloudy haze of material. It can also cause cracking as the expanding pressure pushes out wood fibers.

K&G dries wood down to about 7% moisture in a convential manner, and then right before stabilizing puts the wood through a super low moisture, medium temp kiln with fans. That means that the wood is near zero percent moisture when it goes into the resin, allowing for the best possible product
 
Next post:
What gets in the way of filling the porespace with resin? – WATER

A funny thing happened on the way to the (knife) scales:

Several weeks ago I scored a bunch of basically dried black walnut from a local guy who sources it from southern MN farmland, and turns it in to slab tables (they are gorgeous!). He has a supply of “ends” which are just perfect for knife scales. Although these were “kiln dried” they had been sitting in a pretty uncontrolled garage. So … after cutting the slabs down to 2x2” blocks, some of my “leftovers” were ½” slabs (perfect for the small “dinner” knives I am making for my wife). I decided to see if I could “push” them to total dryness by putting them in to a food dehydrator I have (145F max temp, circulated) – I also wanted to see just how dry they had stayed. Here they are – just a few small pieces:
View attachment 1166144v


(I’ll explain the “138” in a little bit).

Before I put them into the dehydrator, I weighed them. They were 145g (according to my kitchen scale, above). It is not perfect, but probably accurate enough for current purposes. I DO have access to a laboratory scale accurate to 0.0001g – might pull it out later if seems necessary . Anyway, I put these babies into that food dehydrator (think of it as a small but very active “drying box” at 145F). After weighing once in a while as things went, they lost weight, and leveled out (i.e. lost no more weight) after 24 hours at 135g. (the "138" was measured after about 4 hours after taking them out of the dehydrator. they had pulled water out of the air and regained that weight all on their own)

Ok, they lost 10g of water. 10g/145g = ~7% moisture initially. Seems reasonable.

But wait – this is 7% by weight – and water is more dense than (dry) wood. So – what does this LOOK like?

10g of water is 10mL of water. So – this is what that volume of water looks like next to those slabs of wood:

View attachment 1166145

That is a LOT of water relative to the volume of the wood it came out of. I do not know about you – but faced with the visual reality (not just some number like “6%”, I was rather taken aback. Where does all this water go in the wood?

Ordinarily with something like this, we speak about adsorption of water to the surface (or inner surfaces) of something (and yes, the word is “adsorption”, not “absorption”). But adsorption to something is a molecularly thin surface layer – not a lot. However, there is a phenomenon called “capillary condensation”, where liquid water condenses and fills very small spaces (capillaries), even at pretty high temperatures (the explanation for why this is comes from statistical thermodynamics – think lots of big hairy equations … so lets not go there here. Suffice it to say that this is an observed and measurable phenomenon). Also suffice it so say that there is a lot of journal data out there to confirm that capillary condensation is a very active mechanism in moisture retention in wood.

If you have a lot of capillaries, you can hold a LOT of water. The other aspect of capillary condensation is that it is very, very, highly biased towards putting that water into liquid form in the capillaries. Even with very low ambient humidity, the forces involved will “pull” the water out of the air and cause it to condense in those capillaries. This mechanism is, in fact, exactly how many very effective desiccants work. The other thing to know is that the amount of liquid water in the capillaries exists in an equilibrium with the ambient humidity, with the balance of that equilibrium being dependent on temperature (the lower the temperature the more capillary condensation, the higher the temperature the less the capillary condensation). This water will not dry out with time – in fact the wood will actively “pull” water into it, even with very low ambient humidities.

So – going back to our “model pore”, water in the porespace looks something like this:
View attachment 1166146
Where the blue areas are meant to be condensed (liquid) water.

Three points:

1) when you pull a vacuum and pull the air out of the porespace, you will not remove very much of the water: the air will just kind of push the water aside to the walls where it will stay, and then when the air stops rushing by, the water will just “pool up” again in the small pores (remember – the forces creating these liquid water pools are extremely strong).

2) When you try to force acrylic resin into the porespace, you might succeed in “pushing” the water deeper into the pores, but you will not remove it. Ultimately, the volume taken up by this water is pore volume that the resin will not fill.

3) Remember there is a LOT of this liquid water, filling a lot of the porespace. Not good for filling as much of the porespace with resin as you can.

So just how do you go about getting rid of that liquid water in the wood? One way to look at this is from information in the Wood Handbook ( https://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf ) which gives some cool information on water content versus temperature and humidity. One figure they present on equilibrium moisture content is this:
View attachment 1166147

So – you can reduce moisture content by varying either ambient temperature or ambient humidity. Accurately controlling ambient humidity, especially to very low values is HARD (I have done it while characterizing desiccants – it takes a $15-20k piece of equipment, and a lot of tuning of the controllers). So for our practical purposes, that leaves only TEMPERATURE. Lets pick on the 50% RH data (the handbook also gives numerical values). The relationship at 50% RH between equilibrium moisture content and temperature looks like this:
View attachment 1166148

Look closely at that curve. Even if you heat the wood to 240F, you are only going to be able to remove about half of the residual moisture, no matter how long you heat it for.


Now – these are more or less average values. The actual values will depend on species – but this should be enough to make the points. And the points are:

1) If you do nothing to remove moisture in the wood before trying to impregnate with resin, there will be a LOT of liquid water in the pores, and that liquid water will block the resin from filling a lot of the porespace.

2) The only practical way of removing that water is to heat the wood – the hotter the better (without burning the wood of course). Even then, you will never get all of the water out.

3) Capillary condensation is a powerful and fast mechanism. Once you remove the wood from the oven (or dryer) it will immediately start pulling water out of the air. A lot of it. Quickly. So protect it while it cools (zip loc bags with air removed, or better yet vacuum sealed food storage bags. (do NOT put the wood into the resin while hot - the heat will start the resin curing before you can get it into the pores)

One other thing: moisture meters, especially the two-prong conductivity meters, become very inaccurate at low values of moisture (the reason comes from something called “percolation theory” – again coming from the petroleum industry – really cool (you can demonstrate it with M&M’s ) – but that is way beyond the current topic). So … if you want to track the moisture content of your drying wood as you heat it, get a digital scale and let weight loss over time guide you. Timeframes for typical scales or 2x2 blocks will likely (just based on my recent experience) be measured in days. K&G does bake their stuff prior to impregnating with resin, and they use a moisture meter to judge degree of dryness.
You mentioned that it is extremely hard to accurately control humidity. How accurate can a simple heat exchanger system (like in a dryer or AC unit) be? Would this be a viable method of drying in large scale? Is there any way to use a Hygroscopic chemical that wants to pull water more than it wants to stay in the pores? I'm very curious about all this!
 
My short answer is that softer woods like poplar and big leaf maple are easy to do with cactus juice. Once you get to walnut or denser/oilier, you won’t do a great job. It’s not cheap to do it yourself, and getting set up cost me $500.00 cdn. I like being able to do my own, as I have a ton of mature poplar on my property, and I have spalted and rainbow varieties. Cross border shipping is a pain too. It works well enough to be an option, but I send out my nicest wood to k&g.
 
My short answer is that softer woods like poplar and big leaf maple are easy to do with cactus juice. Once you get to walnut or denser/oilier, you won’t do a great job. It’s not cheap to do it yourself, and getting set up cost me $500.00 cdn. I like being able to do my own, as I have a ton of mature poplar on my property, and I have spalted and rainbow varieties. Cross border shipping is a pain too. It works well enough to be an option, but I send out my nicest wood to k&g.
I am thinking ahead ... but my guess is that spalted wood also would work reasonably well with home stabilization (lots of big cracks - easier to get the water out and get the resin in). Just a guess though. What is your experience Willie71?
 
If you put wet wood into a stabilizing set up, the water will boil in the vacuum, and it will interfere with the curing of the resin, leaving a sticky, cloudy haze of material.
good point Ben. I was thinking mostly about the moisture in the smaller pores, which can withstand the vacuum (at least for a while - the condensed phase in the small pores is a super low energy state - thats the stat thermo speaking.....)

K&G dries wood down to about 7% moisture in a conventional manner, and then right before stabilizing puts the wood through a super low moisture, medium temp kiln
do you perhaps know how they achieve the low moisture in the kiln? Inject dry nitrogen? Use a desiccating coalescing dryer? If so, that is NOT the kind of equipment a home user will want to consider - the dryer column itself is ~$2k - not to mention the compressed air/dry nitrogen source.....
 
You mentioned that it is extremely hard to accurately control humidity. How accurate can a simple heat exchanger system (like in a dryer or AC unit) be? Would this be a viable method of drying in large scale? Is there any way to use a Hygroscopic chemical that wants to pull water more than it wants to stay in the pores?
Two questions. No way a dryer (clothes dryer???) or AC unit will work to reduce RH enough. First, a clothes dryer heats only - it does not actively remove moisture. An AC unit does actively remove moisture, but not nearly efficiently enough. I picked on the 50% RH line for the above plot for a reason - that point is a typical target that most home and office HVAC systems can hit (depending on conditions). To get lower than that you run in to a lot of issues: moisture is everywhere: it is in the air, it is adsorbed onto surfaces in a chamber, it leaks in to a chamber through cracks & openings, etc. The lower you actually get the moisture in a chamber, the more "actively" it will penetrate & contaminate that system (basic mass transport: stuff moves from areas of high concentration to areas of low concentration. the greater the difference, the greater the amount of movement). Take a look at the plot I gave above:
upload_2019-7-22_16-8-38.png
to get to really low moisture content, you need to reduce the relative humidity to something like 10-20% or so. This is extremely difficult because of the reasons I mentioned above. to get there, you need one or a combination of the following: injection of dry nitrogen, a very, very well sealed cabinet (think metal, all seams welded shut, a door with high quality rubber gaskets), AND you need an active drying mechanism with recirculation of the air in the chamber continuously through the dryer. The typical ones I am familiar with are called "Pressure Swing Adsorption Dryers:
upload_2019-7-22_16-12-43.png
These are really cool ... and really counter-intuitive, as they use the air in the chamber itself to "regenerate" one column while the other column acts as a coalescing (drying) column. But they are expensive (~$2k), and need either oil-free compressed air or dry nitrogen to run (they are also really noisy!). I have used those to get down to something like 5% RH ... but that was with a lot of tweaking of controllers, NOT opening the door to the chamber, and it took quite a few hours to get the chamber down to that level. Just not practical for the home.

Now - your question about using some sort of other desiccant is a good one. In theory, of you can find a desiccant that has a greater affinity to water than wood, you could do this. Such desiccants likely exist - but one would need to research and compare what are called "adsorption isotherms" to choose the one to use. Like above, you would need an extremely well sealed container/chamber. you would also likely need quite a bit of the desiccant. you would also need to bake (at high temps) the daylights out of the desiccant to drive moisture out of it before using (same deal as with wood), AND you had better get it right from the oven to the well-sealed container in very little time, and get that chamber sealed up. Remember - the better the desiccant, the faster it will adsorb moisture (and in so doing degrade its ability to pull more moisture out of the air (or to compete with the wood). The work I did had to do with characterizing desiccants used to place into implantable defibrillators (and similar hermetically sealed devices with electronics inside). The "working time" of the desiccant between being pulled out of the oven, placed into the device, then getting the device hermetically sealed was measured in minutes. That gives you an idea of how fast you would need to act. But it would work in theory :)
 
I am thinking ahead ... but my guess is that spalted wood also would work reasonably well with home stabilization (lots of big cracks - easier to get the water out and get the resin in). Just a guess though. What is your experience Willie71?

Spalting wood is pretty soft, on the verge of rotting. It’s almost like picking up styrofoam. If it’s too far gone, it just crumbles.
 
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Could you use the combination of heat and vacuum? Something like: pre-heat wood in an oven; drop into empty resin container in the vacuum chamber and pull a partial vacuum; keep the chamber warm (60-80c?) for an hour or two, then leave to cool; open it up, add resin, and immediately pull vacuum again.
I don't know if it would make a difference, and it would be slow, but it wouldn't require much additional equipment. heating the chamber might even be overkill, residual heat in the wood might be enough
 
Could you use the combination of heat and vacuum? Something like: pre-heat wood in an oven; drop into empty resin container in the vacuum chamber and pull a partial vacuum; keep the chamber warm (60-80c?) for an hour or two, then leave to cool; open it up, add resin, and immediately pull vacuum again.
I don't know if it would make a difference, and it would be slow, but it wouldn't require much additional equipment. heating the chamber might even be overkill, residual heat in the wood might be enough
It would not hurt ... but I am not at all sure about the timeframes. the small capillaries will "fight" to hold the water, even against a vacuum. Ultimately they will lose ... but it might take a while. My gut tells me (totally shoot-from-hip) that a timeframe of "hours" is too short - probably something like days would be needed. You could test this by weighing the blocks:heat, pull vacuum on them, hold the vacuum, pull out and weigh say twice a day (morning and evening). put back and and pull the vacuum ... repeat. If you are able to do that the data would be interesting! (I do not currently have a vacuum chamber.......)
 
If you are able to do that the data would be interesting! (I do not currently have a vacuum chamber.......)
i don't have one either, and probably won't for a while :\

water is really tenacious stuff, especially at small scales
 
Two questions. No way a dryer (clothes dryer???) or AC unit will work to reduce RH enough. First, a clothes dryer heats only - it does not actively remove moisture. An AC unit does actively remove moisture, but not nearly efficiently enough. I picked on the 50% RH line for the above plot for a reason - that point is a typical target that most home and office HVAC systems can hit (depending on conditions). To get lower than that you run in to a lot of issues: moisture is everywhere: it is in the air, it is adsorbed onto surfaces in a chamber, it leaks in to a chamber through cracks & openings, etc. The lower you actually get the moisture in a chamber, the more "actively" it will penetrate & contaminate that system (basic mass transport: stuff moves from areas of high concentration to areas of low concentration. the greater the difference, the greater the amount of movement). Take a look at the plot I gave above:
View attachment 1166311
to get to really low moisture content, you need to reduce the relative humidity to something like 10-20% or so. This is extremely difficult because of the reasons I mentioned above. to get there, you need one or a combination of the following: injection of dry nitrogen, a very, very well sealed cabinet (think metal, all seams welded shut, a door with high quality rubber gaskets), AND you need an active drying mechanism with recirculation of the air in the chamber continuously through the dryer. The typical ones I am familiar with are called "Pressure Swing Adsorption Dryers:
View attachment 1166313
These are really cool ... and really counter-intuitive, as they use the air in the chamber itself to "regenerate" one column while the other column acts as a coalescing (drying) column. But they are expensive (~$2k), and need either oil-free compressed air or dry nitrogen to run (they are also really noisy!). I have used those to get down to something like 5% RH ... but that was with a lot of tweaking of controllers, NOT opening the door to the chamber, and it took quite a few hours to get the chamber down to that level. Just not practical for the home.

Now - your question about using some sort of other desiccant is a good one. In theory, of you can find a desiccant that has a greater affinity to water than wood, you could do this. Such desiccants likely exist - but one would need to research and compare what are called "adsorption isotherms" to choose the one to use. Like above, you would need an extremely well sealed container/chamber. you would also likely need quite a bit of the desiccant. you would also need to bake (at high temps) the daylights out of the desiccant to drive moisture out of it before using (same deal as with wood), AND you had better get it right from the oven to the well-sealed container in very little time, and get that chamber sealed up. Remember - the better the desiccant, the faster it will adsorb moisture (and in so doing degrade its ability to pull more moisture out of the air (or to compete with the wood). The work I did had to do with characterizing desiccants used to place into implantable defibrillators (and similar hermetically sealed devices with electronics inside). The "working time" of the desiccant between being pulled out of the oven, placed into the device, then getting the device hermetically sealed was measured in minutes. That gives you an idea of how fast you would need to act. But it would work in theory :)
Are there any dessicants that Chemically ABSORB instead of adsorbing? Presumably anything with a high enough surface area is adsorbant, no?

Also, It says here,
https://www.diffen.com/difference/Absorption_vs_Adsorption
That adsorption favors low temperatures. in that case, why would heating improve efficacy? Unless that isnt true (Im sure it's much more complex than what's stated on that page.)
 
My short answer is that softer woods like poplar and big leaf maple are easy to do with cactus juice. Once you get to walnut or denser/oilier, you won’t do a great job. It’s not cheap to do it yourself, and getting set up cost me $500.00 cdn. I like being able to do my own, as I have a ton of mature poplar on my property, and I have spalted and rainbow varieties. Cross border shipping is a pain too. It works well enough to be an option, but I send out my nicest wood to k&g.
Oh man, where in Canada are you? Here in southern ontario we have plenty of spalted beech, walnut, maple, then the *gasp* Soft conifers!

I may be interested in grabbing some poplar from you!
 
Are there any dessicants that Chemically ABSORB instead of adsorbing? Presumably anything with a high enough surface area is adsorbant, no?
you know ... I do not really know. All the highly efficient desiccants seem to be based on high surface area, and thus are based on adsorption mechanisms (zeolites are the classic example). Not everything with a high surface area will be a good dessicant: you also need chemical affinity between the water and the chemical "active sites" on the surfaces of the desiccant (see below)

Also, It says here,
https://www.diffen.com/difference/Absorption_vs_Adsorption
That adsorption favors low temperatures. in that case, why would heating improve efficacy? Unless that isnt true
Hmmm - to try to make accessible lets try this: for adsorption of water to a surface there needs to be some sort of chemical attraction between the water and specific molecular sites on the desiccant (most typically "hydrogen bonding"). BUT, there are ultimately a limited number of these available "active sites" on the surfaces. When they fill up with water - no more adsorption to the surface ("capillary condensation" somewhat different - so lets leave that out for now). When you get/buy a desiccant, it will ALWAYS have some water occupying those active sites (remember the attraction is strong). The presence of that water interferes with/degrades the ability of the desiccant to adsorb more water. So to use the desiccant most effectively, you need to get RID of that adsorbed water before you put the desiccant into the place where you want it to do its job. You get rid of that water by heating the daylights out of the desiccant (some will call this "activating" it). That heating drives off that water, making available more of the "active sites" for when you really want the desiccant to do its job. zeolites are basically ceramic and can "take the heat", so zeolite based desiccants are heated to VERY high temperatures to drive off that initial water that comes with them. Does that make sense????
 
you know ... I do not really know. All the highly efficient desiccants seem to be based on high surface area, and thus are based on adsorption mechanisms (zeolites are the classic example). Not everything with a high surface area will be a good dessicant: you also need chemical affinity between the water and the chemical "active sites" on the surfaces of the desiccant (see below)


Hmmm - to try to make accessible lets try this: for adsorption of water to a surface there needs to be some sort of chemical attraction between the water and specific molecular sites on the desiccant (most typically "hydrogen bonding"). BUT, there are ultimately a limited number of these available "active sites" on the surfaces. When they fill up with water - no more adsorption to the surface ("capillary condensation" somewhat different - so lets leave that out for now). When you get/buy a desiccant, it will ALWAYS have some water occupying those active sites (remember the attraction is strong). The presence of that water interferes with/degrades the ability of the desiccant to adsorb more water. So to use the desiccant most effectively, you need to get RID of that adsorbed water before you put the desiccant into the place where you want it to do its job. You get rid of that water by heating the daylights out of the desiccant (some will call this "activating" it). That heating drives off that water, making available more of the "active sites" for when you really want the desiccant to do its job. zeolites are basically ceramic and can "take the heat", so zeolite based desiccants are heated to VERY high temperatures to drive off that initial water that comes with them. Does that make sense????
Yes. But you would still do the actual desiccating under lower temps?
 
es. But you would still do the actual desiccating under lower temps?
Yes. I realized I did not answer your questions re. absorption vs adsorption versus temperature. "absorption" as you are reading requires the diffusion of molecules through a solid to disperse and "fill" the solid (not really the right word .. but hopefully close enough for now). "adsorption" to a surface requires the diffusion of the molecules through air (or through a gas - or sometimes a liquid) to reach the surface with the active sites. (technically your water softener, if you have one, is based on adsorption of ions to the surfaces of the resin in the softener's column). Now - the rates of diffusion are always temperature dependent - increasing with increasing temperatures. Also, the rates of diffusion through a solid, as compared to through a gas, are MUCH slower. combine those two points, and the rates of adsorption, which are based on the ability of a molecule to make it to the surface, are maintained at a higher level as the temperature goes down (as compared to diffusion through a solid - i.e. absorption).
 
Yes. I realized I did not answer your questions re. absorption vs adsorption versus temperature. "absorption" as you are reading requires the diffusion of molecules through a solid to disperse and "fill" the solid (not really the right word .. but hopefully close enough for now). "adsorption" to a surface requires the diffusion of the molecules through air (or through a gas - or sometimes a liquid) to reach the surface with the active sites. (technically your water softener, if you have one, is based on adsorption of ions to the surfaces of the resin in the softener's column). Now - the rates of diffusion are always temperature dependent - increasing with increasing temperatures. Also, the rates of diffusion through a solid, as compared to through a gas, are MUCH slower. combine those two points, and the rates of adsorption, which are based on the ability of a molecule to make it to the surface, are maintained at a higher level as the temperature goes down (as compared to diffusion through a solid - i.e. absorption).
Understood, So it's not that adsorption is faster at low temperatures, it simply performs better than absorption at those temperatures. At higher temps, adsorption would still be faster, correct?

Something that may be worth trying is freeze drying. If it's good enough for NASA...
 
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Oh man, where in Canada are you? Here in southern ontario we have plenty of spalted beech, walnut, maple, then the *gasp* Soft conifers!

I may be interested in grabbing some poplar from you!

I’ve been harvesting wood this month, and putting aside some nice stuff for guitars, tables, and knife handles. I’ve got a few trees with trunks over 24” wide!

This is my first stack to start drying. The bottom is a silver maple planted in 1962, by my uncle who passed away about a year and a half ago. I’m making at least three guitars out of this, as we have several pro musicians in the family. The top 6 pieces are the poplar. I cut down two more threes already, but the bases are still in the wooded area, as it rained heavy since I cut them. I’ll get them out this week. The bases almost all have spalting and or heavy curl. The top two pieces are crotch poplar for hopefully one piece guitar bodies. I’m going to assemble a passive solar kiln for drying as time permits.

790D384B-7241-49D6-87E8-2F09F4F6551C by Wjkrywko, on Flickr

I’m in Alberta.
 
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Understood, So it's not that adsorption is faster at low temperatures, it simply performs better than absorption at those temperatures. At higher temps, adsorption would still be faster, correct?

Something that may be worth trying is freeze drying. If it's good enough for NASA...
Correct!

Problem with freezing is you may split the wood as the ice expands?
 
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