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· Moderator / Gold Bullet Member
11,539 Posts
AHHH One of my favorite subjects! From a conservation point of view, I am skeptical about putting (old) leather in an oven at such temperatures. Freezing seems a more logical approach as it does not strip the humidity that is in the leather.
Ever hear of "freeze dried" food? If what you say has any validity..I wonder how it gets "dried"?
Let's discuss the freezing method..almost no home freezer in American households is cold enough to adequately freeze ice cream much less kill encapsulated mold spoors. Freezers in most households are at 0 degrees F or warmer.
A report given by Sandra Nyberg of Stanford University in 1987, states that temperatures well below freezing will keep mold and mold spores dormant, but will not kill mold or the spores.
What causes freezer burn and dries foods? Air. Not the freezing but air circulation. Freezing is not used in any commercial aspect to kill anything I know of.

Known Methods of Sterilization

Heat: most important and widely used. For sterilization always consider type of heat, time of application and temperature to ensure destruction of all microorganisms. Endospores of bacteria are considered the most thermoduric of all cells so their destruction guarantees sterility.

Incineration: burns organisms and physically destroys them. Used for needles , inoculating wires, glassware, etc. and objects not destroyed in the incineration process.

Boiling: 100o for 30 minutes. Kills everything except some endospores (Actually, for the purposes of purifying drinking water 100o for five minutes is probably adequate though there have been some reports that Giardia cysts can survive this process). To kill endospores, and therefore sterilize the solution, very long or intermittent boiling is required.

Autoclaving (steam under pressure or pressure cooker): 121o for 15 minutes (15#/in2 pressure). Good for sterilizing almost anything, but heat-labile substances will be denatured or destroyed.

Dry heat (hot air oven): 160o/2hours or 170o/1hour. Used for glassware, metal, and objects that won't melt.
The protocol and recommendations for the use of heat to control microbial growth are given in Table 1.

The above is proven protocol. Heat kills. Guaranteed.

Now we talk about another very valid concern and that is heat in a dry oven and it's extracting moisture from vintage leather. Yes..this is very true and it does exactly that. There is no avoiding this fact. On the one hand extracting moisture temporarily can be seen as a good thing. Moisture is what enables our enemy to thrive. Mold is being attacked with heat and it's necessary ingrediant for life..water.
I want to mention here experience learned the hard way..DO NOT attempt to trap moisture around the leather. It will BOIL it, cook it and DESTROY it. The leather must be left out in the open oven with nothing covering it.
I have baked many dozens of holsters using my method and have yet to experience any drying out of the leather. Even VERY dry leather is like a sponge, once out of the oven it will again absorb ambient moisture levels in the air. There is no preventing this unless you prevent the leather from air contact. Eventually moisture that escaped due to high temps returns.

Let's discuss temperatures..It's known that almost no living thing can withstand 200 degrees and above. This being the case I formulated my mold eradication method to start with an oven temperature of 350 F, shut the oven off and insert leather. This method is for safety of the leather item. It is possible to forget to shut off the oven if not shut off upon insertion. Long term temperatures of 350 will kill your leather. With my method the leather item will NEVER reach anything close to 350. If left until the oven cools I predict a top temperature on the interior 3-4 layers of leather to reach 200 F. If a soaking temp. of 200 F is reached into the very deepest interior you can be assured that any mold present has died. Leather being leather I made my method quick easy and foolproof if directions are followed. As you might notice above... the industry standard is Dry heat (hot air oven): 160o/2hours or 170o/1hour. I believe my method achieves this at a minimum.
The "white mold" seen on leather is the mold flower releasing many more mold spores. By the time this stage is reached..your environment. air, surfaces..furniture, storage, EVERYTHING anywhere near this infected leather will be covered in mold spores. Laying in wait to attach themselves to a viable surface environment to begin more growth cycles.
You say..I tried the heat method and the mold came back..Did it? Mold is actually everywhere. A LOT is in your car. You eat, drop some crumbs, wet shoes, carpet..MOLD grows. You likely don't notice it but it's there. Like germs mold is hard to see in most instances. Mold grows in covered food in the fridge..It was on your food while you were eating it..
So after heat treatment your leather is once again brought into a mold rich environment. You MAY have killed it all.... then it is simply re infected.
Leather can be an ideal bed to grow mold. It is pourous, accepts ambient moisture and has available nutrients. Storage is key to keeping in under control. We have mentioned the "drying out" of the dry heat method and I firmly believe dryer leather is WAY better than moist leather. Moisture is a proven mold starter. Within parameters..a dry leather is much preferred to control mold growth.
Now we go on to other control or eradication methods advocated by some..
Baking soda
Cleaning disinfectants, like Lysol
White vinegar.
Or God forbid..formaldehyde? If you have EVER smelled formaldehyde you will never forget it. I once bought a few holsters that a well known collector had stored mothballs in..These things CAN NEVER BE REMOVED. You have stored, sprayed, daubed, slopped or poured it on.. NEVER TO BE EXTRACTED.


These precious artifacts are in our care for a very short time actually..Give what you do to preserve them at least more than a passing thought. I have thought this through for a decade and after much experimentation I believe the LEAST damaging way to try and salvage a moldy piece of gear is heat. But I am certainly not the end all to this discussion..I just know what works for me.

· Moderator / Gold Bullet Member
11,539 Posts
I think my mistake may have been using an electric oven. Is your process in a gas fired oven? Your "mistake" ?? What happened?

I have used both. Either one is a "dry heat" and IMO makes no difference whatever.

Which is worse, a holster eat up by mold or one with a little smell on it. Harry..neither one is my preference. That's why my method is such a good one. Not only that but when the leather comes out and is still warm the verdegris can be simply wiped off with a terry cloth. An added benefit.

Seriously, Jerry has fixed more leather than most of us will ever see. Ron, very kind of you to say but even so I do not consider myself any kind of authority. I remain now and always will be a student. My claim to any knowledge of the subject comes with some experience and research into how to kill the stuff.
Like Don says.. No one ever reads the stickys!
He repeated the same experience I had early on with boiling...That's what led me to dry heat. I hate like sin he had that happen. But he did.

So we live and learn. I wish good luck to any collector out there with a mold problem. Read and research all you can before doing something irreversable. Could be expensive and ruin a historical artifact you were wanting to preserve.
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