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Discussion Starter · #1 · (Edited)
I have read a lot of people recommending that wood be sterilized with an autoclave. I don't think that this will work as well as everyone seems to think it will - unless it is in the autoclave for a very long time.

Autoclaves are great for sterilizing small metal objects and I'm sure that there has been a multitude of data collected on using autoclaves to sterilize small metal objects, but has there been any data collected for thick chunks of wood?

I think that using an autoclave to sterilize wood is a lot like trying to cook a turkey in 1 hour by raising the oven temperature to 600°. The turkey will burn on the outside but will still be raw on the inside. This is because it takes time for the heat to conduct through to the center of the turkey.

The coefficient of thermal conductivity tells how fast heat will be conducted through a material. Here is a small list of coefficients of thermal conductivity (in units of W/(m*K) ):

iron: 80
carbon steel: 54
stainless steel: 16

salmon:0.50
beef: 0.46

hardwoods: 0.16
softwoods: 0.12
cork: 0.07

(source)

I think that the numbers above mean that hardwoods will take 100 times longer (16/0.16=100) to heat through to the center than stainless steel (like most medical instruments sterilized in an autoclave). Most of the wood we use is also a lot thicker than medical instruments and will take longer to sterilize for that reason also.

I got out my heat transfer textbook to try to calculate how long it would take to "fully cook" a piece of wood, but heat transfer calculations are even worse than I remember, and I got nowhere on the calculation. Does anyone have any test data or calculations on heating wood (not skinny, metallic medical devices) through to the core in an autoclave vs. home oven or simply boiling in water?
 

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One of the benefits to using some form of autoclave is that unlike boiling/baking, the (slightly) increased pressure helps the steam to penetrate the cracks/crevices in the wood. These areas are some of the more problematic ones as they can be insulated from exposure to heat penetration when boiled or baked allowing unwanted organisms to survive simple baking/boiling.

In addition, I don't remember anyone saying that autoclaving wood is going to be fast.

With respect to other materials, autoclaves are used for sterilizing materials that tend to be poor heat conductors (like straw and sawdust) with good success...although time required to totally sterilize the material increases with quantity.

As I've said before with respect, the best thing a person can do with wood/bark is to let it throughly dry out, and then scrub it with a stiff brush and hot watere....

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Ed
 

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I got out my heat transfer textbook to try to calculate how long it would take to "fully cook" a piece of wood, but heat transfer calculations are even worse than I remember, and I got nowhere on the calculation. Does anyone have any test data or calculations on heating wood (not skinny, metallic medical devices) through to the core in an autoclave vs. home oven or simply boiling in water?
Dense sawdust mixtures can be used to approximate wood... and depending on the quantity it takes between 2 and 4 hours to sterilize wood (Stamets, various publications). Sterilization isn't really necessary as long as the temperature reaches a point that macrofauna is killed as opposed to say spore forming bacteria. As I noted above, the slight increase in pressure helps the heat penetrate the crevices and holes that can remain protected when boiling and baking.

We have seen people attempt to boil/bake wood without success... See the thread where the person baked a thin piece of wood and then when cutting it to fit discovered an ant colony survived the treatment.....

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Ed
 

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I autoclave all wood. And contrary to your statement, I also autoclave huge volumes of liquids inside polymeric carboys that have crappy heat transfer. You put a data logger inside a 50l carboy and see what temp it gets to..... Always 121c after a 60 min liquid sterilizarion cycle. When autoclaving wood, ALWAYS use the liquid cycle, not the dry cycle you use for metal goods or empty lab glassware. At the amount of pressure generated the steam penetrates the wood extremely well.
 

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Discussion Starter · #5 ·
The ant story is exactly why I was "concerned" about autoclaves. The wikipedia article for autoclaves, lists a cycle at 121°C for 15-20 minutes. I am pretty sure that this would not have killed the ants in the wood. But using the liquid cycle on the autoclave, or some other way of leaving the autoclave on for a few hours makes me a lot more confident in appropriateness of using an autoclave to sterilize wood.

The high pressure has been mentioned as helpful for sterilizing the cracks. I do not believe this is correct. Pressure will push a gas through a small area only as long as there is a pressure difference. When the steam is pushed into the autoclave the steam will be at a higher pressure than the air in the crack. As the steam enters the crack it will compress the air and raise the pressure of the air before the steam has traveled very far down the crack.

Using an autoclave with a vacuum pump to remove the air will help, but the small amount of steam forced into the crack will not have enough heat to sterilize wood all the way down to the bottom of the crack. It will still take a lot of time for the heat to get down to the bottom of the crack.

Sterilizing sawdust in an autoclave definitely validates the autoclave effectiveness on driftwood. How long did the sawdust have to remain in the autoclave, and how deep was the sawdust?

I'm not trying to make waves, I am just concerned that autoclaves are portrayed on this site as being tons better than boiling and baking. But I think that they still require more time than any of us have realized - and I have not seen any discussion on this site about cook time in an autoclave (although I haven't looked). But I have read many comments along the lines of: "Baking and Boiling are not that effective. Using an autoclave is the only sure way to sterilize your driftwood."
 

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Discussion Starter · #6 ·
Frog party, the carboy itself does have a very low thermal conductivity, but the carboy walls are quite thin. The liquids inside will have a much higher thermal conductivity and will probably have convection heat transfer also.

Does your data logger have a thermocouple? I'd be very curious to see what happened if you were able to measure the internal temperature of a piece of wood.
 

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Frogparty's post has some great info. I try to autoclave most of what goes into my tanks (wood, leaf litter, coco huts, sometimes I recycle substrate that way), and I find that the 30min liquid cycle is sufficient for most applications. When autoclaving large volumes of liquid you may have to extend that to 60min, as Frogparty suggested.
Unfortunately, large pieces of wood may need even longer tha that. I autoclaved a stump once (about 15 inch diameter, 25 inch height) for 45min on liquid, put it in one of my tanks. Didn't cause any problems until about 6 months later, when a whole army of millipedes came crawling out of it. So obviously, the temperature in the center never reached 121C. In this case, submerging the stump in boiling water may have been more effective as it might have driven the gas out of the cracks and crevices deep inside the stump. Bottom line: regular autoclaving works great for most stuff but you do have a point that thick wood may need to be boiled or autoclaved for a really long time.
 

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Yes, we have a thermocouple. Ill hook it up next time I run some wood in there.


To bring up a more relevant point here.....how many froggers ACTUALLY have ready access to an autoclave? Let alone an autoclave big enough to actually put a large piece(s) of wood into?

An equally constructive discussion would be "How do froggers closely approximate the effectiveness of an autoclave at home?"
Large pressure cookers are pretty much the only at home option, and workwell. From my years cultivating mushrooms and sterilzing sawdust, grain spawn, etc etc I know that 90 minutes @15psi in a good pressure cooker is reliably effective at killing off pretty much anything you need to worry about
 

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The high pressure has been mentioned as helpful for sterilizing the cracks. I do not believe this is correct. Pressure will push a gas through a small area only as long as there is a pressure difference. When the steam is pushed into the autoclave the steam will be at a higher pressure than the air in the crack. As the steam enters the crack it will compress the air and raise the pressure of the air before the steam has traveled very far down the crack.
Okay are you trying to BS me?
The only time you would get pressurization of a atmosphere as you describe is because of some form of moveable membrane between the steam and the gas behind the membrane (and then we would have to ignore the fact that the gas behind the membrane would increase in temperature with respect to the pressurization). With respect to the cracks, as the steam is forced into the crack due to the increased pressure, it is going to mix with the atmosphere already in the crack transferring heat with the result that the atmosphere inside the crack also increases in pressure.

Sterilizing sawdust in an autoclave definitely validates the autoclave effectiveness on driftwood. How long did the sawdust have to remain in the autoclave, and how deep was the sawdust?
As I noted above, with the reference, between 2-4 hours depending on the quantity..

I'm not trying to make waves, I am just concerned that autoclaves are portrayed on this site as being tons better than boiling and baking. But I think that they still require more time than any of us have realized - and I have not seen any discussion on this site about cook time in an autoclave (although I haven't looked). But I have read many comments along the lines of: "Baking and Boiling are not that effective. Using an autoclave is the only sure way to sterilize your driftwood."
Baking and boiling aren't that effective... in fact instead of attempting to boil and bake, the same amount of effort should just be used by using a stiff brush and some hot water......

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Ed
 

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Okay are you trying to BS me?
The only time you would get pressurization of a atmosphere as you describe is because of some form of moveable membrane between the steam and the gas behind the membrane (and then we would have to ignore the fact that the gas behind the membrane would increase in temperature with respect to the pressurization). With respect to the cracks, as the steam is forced into the crack due to the increased pressure, it is going to mix with the atmosphere already in the crack transferring heat with the result that the atmosphere inside the crack also increases in pressure.



I think what he means is that when the autoclave chamber is pressurized pockets of air may remain inside the wood. Thus, heat exchange will be slow.
 

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Discussion Starter · #11 ·
Okay are you trying to BS me?
The only time you would get pressurization of a atmosphere as you describe is because of some form of moveable membrane between the steam and the gas behind the membrane (and then we would have to ignore the fact that the gas behind the membrane would increase in temperature with respect to the pressurization). With respect to the cracks, as the steam is forced into the crack due to the increased pressure, it is going to mix with the atmosphere already in the crack transferring heat with the result that the atmosphere inside the crack also increases in pressure.
I was not trying to say that there would be no mixing or diffusion of the air and water vapor - only that the diffusion would happen a lot slower than the equalization of the pressure -especially in a deep, skinny crack. And I think that the pressure in the crack will have equalized with the pressure outside the crack long before any of the heated water vapor reached the bottom of the crack.

I checked the wikipedia article about autoclaves again and learned that the autoclave works at 15psi above ambient. This is higher than I thought it was. Running some quick numbers in my head (which is never a good idea) and ignoring diffusion, the pressure would push halfway down a rectangular crack and 2/3 of the way down a triangular crack (based on the ideal gas law). This is actually farther than I thought it would push.

frogparty said:
To bring up a more relevant point here.....how many froggers ACTUALLY have ready access to an autoclave? Let alone an autoclave big enough to actually put a large piece(s) of wood into?
That is a very good point and makes this post mostly just academic.
 

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Discussion Starter · #12 ·
I think what he means is that when the autoclave chamber is pressurized pockets of air may remain inside the wood. Thus, heat exchange will be slow.
You beat me to it and said what I was thinking a lot more clearly.
 

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I realise I may be dumbing down this conversation. Feel free to ignore this but...

The colony of ants and the millipeds. Couldn't they been avoided by drilling holes deeper or through the wood to help the heat (and pressure if using a pressure cooker or autoclave) reach the center and harder to reach areas. Or even reveal what is in the center?

Is there a coreing bit that you could
Just push the plug back in if you are concerned about the finish?
 

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Discussion Starter · #14 ·
The colony of ants and the millipeds. Couldn't they been avoided by drilling holes deeper or through the wood to help the heat (and pressure if using a pressure cooker or autoclave) reach the center and harder to reach areas. Or even reveal what is in the center?
Drilling holes would help get heat inside, but very little. Your driftwood would have to look like swiss cheese before it would have much affect.

In my opinion, your best weapon is to cook it for a long time (whether you are boiling, baking or autoclaving).
 

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... and in a first for me ...

First time ever I've moved a topic from Beginner to General.

I've moved hundreds from General (and elsewhere) to Beginner, first time ever I think a topic should be moved up.

s
 

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I was not trying to say that there would be no mixing or diffusion of the air and water vapor - only that the diffusion would happen a lot slower than the equalization of the pressure -especially in a deep, skinny crack. And I think that the pressure in the crack will have equalized with the pressure outside the crack long before any of the heated water vapor reached the bottom of the crack.
Okay, just the descriptive comment about how the gas would be compacted thus resisting the steam and being compressed by the steam, made it look like you were either BSing, or didn't understand gas mechanics.....

The pressure in the crack is going is going to be the same as the pressure inside the container from the beginning... It will be at atmospheric pressure to start and will rise at the same rate as the pressure in the autoclave/pressure cooker.... A crack that is sufficiently deep and thin would also resist penetration by boiling water and dry heat (at atmospheric pressure) much more efficiently than it would resist saturated steam. This is one of the issues with using boiling water and/or an oven as the structure of a crack can inhibit the penetration of water or dry heated air much more efficiently...

I checked the wikipedia article about autoclaves again and learned that the autoclave works at 15psi above ambient. This is higher than I thought it was. Running some quick numbers in my head (which is never a good idea) and ignoring diffusion, the pressure would push halfway down a rectangular crack and 2/3 of the way down a triangular crack (based on the ideal gas law). This is actually farther than I thought it would push.
It should also be noted that many unwanted organisms, are probably not going to tolerate (very well) being exposed to pressures in excess of 15 atmospheres...This is not only going to affect dissolved gases in the tissues, but it will impact the pressure of the fluids in the tissues....

Actually, people can simply purchase a a big pressure cooker if they want to do larger pieces of wood... (a propane burner can be used for those that don't fit on a stove)... I have one that not counting the dials is about 24 inches high... It barely fits on my stove (and I purchased a propane burner (those for turkey fryers work well) for future use..... Smaller pressure cookers can be used for those who want to autoclave leaves or other small batches... The prices aren't excessive for them....

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Ed
 

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You can run most household pressure cookers to 20 or even 25 psi for some of the larger units. Plenty of oomph to do what needs to be done.

There are also plenty of large/oversize autoclavable bags out there so those who are very concerned about sterility can rest assured that their wood/leaves/substrate will remain sterile for long periods of time.

I take access to an autoclave for granted for sure. The one I use every day is 2 metersx 1 meter inner dimensions, and achieves an inner temp of 128-131C. I Guarantee I dont worry about contaminants much.

I have some manzanita branches to run soon, so Ill hook up my thermocouple and run some numbers
 

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Stupid question. I'm quite capable of them.

I've seen no mention of microwaves.

Where do they fit in sterilizing items for a viv?

s
They function by exciting polar molecules which generates heat which generates steam... this means that the maximum temperature is going to be the boiling point of water at that elevation. The other drawback is that the more that is placed into them, the greater the possibility that you will not have even heating which can allow unwanted organisms to survive the exposure.
For relatively small amounts of materials that contain water they are fine, as with boiling water and heating in an oven, larger and/or drier materials and/or thicker materials may not get hot enough to kill unwanted organisms.

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I am quite curious:

1) I assume that these procedures (with which I admit I am unfamiliar) are used for for wood from outside. Are these necessary for wood from pet vendors?

2) What does this do the physical character and integrity of the wood? Heat treated orchid bark does not hold moisture as well as as the fresh stuff. Is this relevant?

3) I am with Ed on hot water and a brush. I don't use wood from outside, but I may reuse a nice piece of wood in another tank--hot water, brush, air dry. I would ask, though, how do deal with burrowed critters if one does not have access to an autoclave? Drill holes and place in oven?
(I do compost old substrates, for use in outdoor growing.)

4) Is the square/cube law relevant here? Does the size/shape of the object dictate a preferred procedure?

5) Finally, and I am asking a question, not making a statement: Are we sure that all these sterilization procedures are mandatory? Is there serious literature on this?

--I rinse the tank, the LECA, the coir, my ABG, the magnolia leaves, the ghost wood, etc prior to set up;
--Never go from tank to tank w/o washing hands first;
--The only thing I may do is, if one big lizard does not want all his superworms or dog food or cheddar cheese goldfish or ravioli or marble cake, is I may let another lizard polish 'em off; sometimes they will eat from communal dishes (I won't give a dog's meal to a lizard, or a lizard's to a dog, though). Won't do this with amphibians.
--Of course, all water bowls are cleaned daily with HOT water, and I only use aged, conditioned water.

Now, I have:

--Bred Corythophanes, Anolis cybotes, green Physignathus, Mabuya (=Eutropis), Elgaria;
--Successfully raised lots of guys from hatchling or metamorph stage;
--Had one water dragon 14 1/2 years;
--Virtually all my hylids and rhacophorids live long lives (I've had my Litoria since August 1998);
--My Tiliqua was born December 5, 1999;
--My Aussie WD was born August 2001;

Please explain what, if anything, I am doing wrong. Or does amphibian husbandry really require a greater degree of fastidiousness? Or is it more necessary, the more extensive the collection? E.g., people with 32 tanks need be extra careful?
 
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