Risks of humidified or heated storage

// Published March 24, 2020 by mgualt

As a result of my storage experiment posts, especially the one detailing the Hotbox storage system, I’ve received a lot of emails and seen some commentary (good and not so good) about it. There is no shortage of pu-heads with very strong opinions on how you should store your tea, so forgive me for adding to the noise: I’m reporting the results of my tests, and you can draw your own conclusions.

Storage goals

In my opinion, it makes no sense to prescribe a storage system unless you know what you are trying to achieve. What is the goal of storage? I outlined my goals at the start:

  • to maintain: pungency, resin, sweetness, energy…
  • to reduce: harsh bitterness, greenness, rough astringency,
  • to develop: emergent flavour, aroma, deepening energetic effects.

I think that most newcomers to puerh, especially if they are following the strategy of buying newly-made tea and hoping it improves with age, will have goals overlapping with those above. If you have well-aged tea and don’t want it to change, I believe that heated (and certainly humidified) storage is not appropriate.


Equally important, since storage is a long game, is whether your system is sustainable: for me this means

  • no maintenance required, e.g. during long absences
  • minimize risk of damage, for example by mold, pests, smells, drying
  • maintain individuality of teas, no mixing of aromas/tastes
  • cheap, energy efficient

Red flags of home storage

The first two points above (no-maintenance and risk of damage) are where most storage solutions go wrong. The following are some of the questions collectors should ask themselves.

  • If you leave your storage alone for 6 months, and if the power goes out, will anything bad happen? If so, your storage is risky.
  • Is there any chance at all that something could tip over and wet your tea? If so, your storage is very risky.
  • Are you keeping your tea near fragrant woods (yes, this includes pine, any new wood, untreated wood etc.)? Near the kitchen? In the garage? Without a seal to keep out smells? Then you need to rethink your storage. Do you vape/smoke/use incense? With tea not sealed away? Again, very risky.
  • Do you heat or air condition your living space? Can this air can travel to your tea? Do you live in a very dry atmosphere? Then you are risking drying out your tea.
  • Is it possible for bugs to get into your storage? Can bugs get through cardboard, fabric, or bamboo? (Hint: yes they can)
  • Is your tea exposed to an active humidity source? If so, what would happen if all the water in that humidity source were released? Would that lead to mold? If so, you are rolling the dice in the long term.

Rationalization of risky choices: True story, bro!

Most collectors are well aware of the red flags listed above. Nevertheless, many somehow find a way to rationalize taking on high levels of risk. Here’s an example of a train of thought I’ve encountered (I have nothing against crock storage, this is just an example):

  1. I want to use a crock.
  2. But my apartment is too dry, so I must add humidity; add Boveda packs.
  3. But the crock is not sealed, so I must add a new Boveda pack each month.

What is potentially wrong with the above? The expense of Boveda packs. The fact that monthly maintenance is not sustainable. The risk of added humidity in an open / uncontrolled system. The fact that smells can get in. The fact that bugs can get in. All of these are unnecessary risks. But the main problem with the above is that we began with an arbitrary choice to use a crock. Perhaps your choice of storage solution should be adapted to your living situation and not the other way around. Here is another example I’ve encountered:

  • Puerh tea improves with age: it’s like wine!
  • Let’s store it in a wine cave!
  • The wine cave has wooden shelves, so we’ll use cardboard boxes to protect the tea.

What is potentially wrong with the above? Perhaps a better question is if there is anything right about it… Wine caves are too cool and too damp, and they have a distinctive musty odor. Cardboard is basically a porous sponge, and offers little protection against smells or mold. The wooden shelves will impart wood taste to the tea. And of course, besides corrupt politicians you can also find bugs in wine caves. In this case, the collector was aware of all these risks, but rationalized using this system because they were enamoured by their pet theory about tea and wine. Perhaps the lesson is: don’t let your pet theory overrule known risk factors.

Main risks of hotbox storage

Over the three years that I’ve been running the hotbox, I’ve identified three issues which people should be aware of.

1. Misunderstanding and Mold

The first issue is not really an issue with the hotbox as I’ve described it here, but a misunderstanding about it. There should be no added humidity to the mylar bags while they are in the hotbox or in long-term storage. Some people seem to think that adding humidity is a minor modification, but it is not. It is very risky to add humidity even if it is a Boveda pack. Boveda packs are slow to react to changes in humidity, and if there is an accumulation of humidity (and if the cake has mold spores, which is often the case), you will get mold. I do use Boveda packs to humidify dry cakes; this is for at most 3 months and I do it at room temperature. Bovedas are not permitted in the hotbox. Here is a moldy example from one of my early tests three years ago with a 69% Boveda pack at 32C:

You can see Aspergillus growing on some of the stems in the overly-humidified cake
Same cake, I believe this is also Aspergillus but the hyphae/mycelium.
The mold easily brushes off since it was found quickly, but the tea would have been ruined eventually.

2. Failure of Mylar bags

The second issue which I had to address on a couple of occasions is the failure of Mylar bags. It is important to use good-quality sturdy Mylar bags for long-term storage. Clear Ziploc bags of any kind are not suitable, they are porous and have notoriously bad seals. But even the best Mylar bags will fail if you manipulate them too many times. If your Mylar bag has a zip closure, the first point of failure is this zip seal (punctures are much less common). Most of my zip bags have survived for 3 years without a zip failure (they continue to maintain the tea’s generated humidity), but some (in my case about 5%) failed and I had to replace them. Some of my bags are heat sealed, and none of these has failed. So, I recommend heat sealing if you are using Mylar. For Mylar zip bags in good condition, however, my tests show they do maintain humidity over multiple years of heated storage. To be clear: if the seal fails, your tea will gradually lose humidity and get too dry.

A better solution, which I find too costly for all my cakes, is to use Onyx stainless steel 23 cm diameter containers (these have a latex seal). These are excellent, and I use them for my more expensive cakes. In fact, for someone with a small collection of only a few cakes, I would strongly recommend using these containers for room temperature storage – they comfortably hold five full-size puerh cakes. These also avoid the “problem” of plastic — if you are concerned about plastic breaking down and somehow entering your tea (I am not, in the case of using Mylar for long-term dry tea storage), you can avoid plastic altogether by using this stainless steel container.

This is the only container I’ve found which I prefer to mylar bags

3. Temperature sensor failure

Another problem I encountered (it happened one time in three years) is that the connection between my temperature sensor and my Heat Outlet Thermostat (Inkbird C206) failed. I think this happened because something hit the sensor plug on an angle and bent it out of shape. This didn’t cause any problem for my tea, it just deactivated the heating system. I still think the C206 is a good unit, and it has even come down in price to 20$, but you should be aware that the connection is not robust enough to be handled roughly.


  1. Matthew Ziegler
    March 24, 2020 @ 4:41 pm

    I have tried a very similar experiment but have disobeyed one of your key tenets; I’ve included 69RH Boveda packs in the mylar bags, but have kept the temp lower at 80-85F. I was worried that higher temperatures would increase the likelihood of temperature differentials in the heated cooler box. Any speculation on how these parameters may compare in outcomes/risk of mold? I’ve only had my box operational for a month or two. Thank you!

    • mgualt
      March 24, 2020 @ 5:03 pm

      I will just repeat what I said in the post. I believe added humidity is an unnecessary long term risk. If the tea is at the appropriate humidity level, it makes no sense to put a boveda in there.

  2. Sebastien
    March 24, 2020 @ 6:48 pm


    What about zisha yixing clay container? I bought one 25cm diameter by 30cm high. It can contain about 8 cakes.
    Is that a good container for long term storage?
    I live in Sud part of Quebec Canada, humidity level between 40% in winter up too 70% summer. Temp around 20 to 27 summer

    • mgualt
      March 24, 2020 @ 7:47 pm

      First, it depends on what you want to achieve. Unglazed clay is porous and the jar is probably unsealed. So you can easily lose humidity. Its impossible to predict, you should measure humidity and temperature. If it stays 60-70 RH and doesn’t go below room temperature, you won’t risk drying out or mold…

      • Sebastien
        March 24, 2020 @ 8:53 pm


        Thank for quick answer! I buy 10+ years old seng. I am not looking to change anything, just want them to age normally without changing much. It is unglazed. It might go down as low as 40rh in winter but I am looking ti wash it with boiled water and let it sun dry this july and then fill it with cake. I am not looking to open it often. Especially not during winter month. And temp never go below 20c and up then 27-29c

        • mgualt
          March 24, 2020 @ 8:58 pm

          Hi Sebastien, I am from Montreal so I know the climate (and the aggressive climate control we have to do). It sounds like your main enemy is them drying out or getting too cold. I think the only thing you need to do is protect them. Unglazed clay without seal is not doing much to protect, in my opinion.

  3. Sebastien
    March 24, 2020 @ 9:17 pm

    Ohh it sad just bought one nice zisha container :-(…
    anyway I will look for others container.

    Do you mix different sheng cakes together?


    • mgualt
      March 24, 2020 @ 9:19 pm

      I have some larger bags and those stainless steel containers I mention above which I use for sheng from a single source. For example I have a Mylar bag of Dayi Sheng cakes and a stainless canister with Biyun Hao cakes.

      • Sebastien
        March 24, 2020 @ 9:39 pm


        I am looking to buy one of those onyx container! I will sell the zisha. I might put different sheng tea together or maybe buy smaller container to split different origin.

        Btw where do you buy your cakes? And what about Camellia sinensis in Montreal?

        • mgualt
          March 25, 2020 @ 8:51 am

          I like the Camellia Sinensis shop and have visited it a few times over the years. But for buying puerh, some friends and I have been organizing group buys from collectors in Taiwan and Malaysia — we wanted to improve on the selection of puerh which is typically available, by working with curators and collectors to access teas which are not usually sold by the larger vendors. Even though our quantities are significantly smaller than normal vendors, we decided to open a website (teaswelike.com) which we operate in our spare time (with very low markup I should add). I would also recommend Wistaria tea house in Taiwan, they have very good puerh (especially the 2003 cakes, which are available online) which are far better value for money than what you find from most of the popular vendors.

          The advice which is often given to new puerh drinkers is to buy tons of samples and use this to educate yourself. This is what I did, but I disagree with this approach. I ended up drinking mostly unhelpful references and wasting resources. Better to buy a small handful (or even one) well regarded reference cake and drink from these for many weeks, learning how these operate. Grow your collection slowly after this. There is a lot of pressure to buy buy buy… My advice is to go slow.

          Maybe more important than finding shops to buy from is to find reviewers you can rely on for good selections. I would recommend Marshaln.com, TeaDB.org, mattchasblog.blogspot.com, deathbytea.blogspot.com, teaintheancientworld.blogspot.com, and half-dipper.blogspot.com. Also check out teaforum.org.

  4. Sebastien
    March 25, 2020 @ 2:42 pm


    I know teawelike! I am planning to buy 2-3 cakes from them soon! Are you still operating? I also just received a 2007 jincha from Emmett YQH, do you know him? As for camellia sinensis, i bought 4 Yiwu 2006-7? Cakes , 1998 Menghai cake and 2007 Laobangzhan cake. Yiwu 2006-7 is very good! Seaweed taste! Laobangzhan ( which I don’t know if it a real one because price was low according to some puer drinker) is very fruity! And 1998 Menghias taste more like earth and wood. Some say that older cake from CS have been most likely traditionally stored which is in humid conditions.

    As for my zisha yixing clay container. I looked at it yesterday. I let tje lid open for a while there is absolutely no smell in it! It have thick wall. I am planning to store tea cake in it. I will wait for july to wash it dry it and them fill it with cake. I will put a hygrometer thermometer in it. I wont open it often during winter months and it will not be close to any window or outisde wall.

    If it get to dry, I will put a 320g 62% boveda pack.

    It a big gamble because it will hold a thousand dollar worth of tea. But I am positive that it will work!

    • mgualt
      March 25, 2020 @ 2:56 pm

      That’s good. Yes, we are still operating. The YQH Jincha is pretty good, I have a couple of them. Good luck with the clay storage, just make sure that whatever system you use, you can keep it going for years…

  5. MattCha
    March 26, 2020 @ 10:56 am


    I have had storage issues with clay storage ( it’s good in Asia not so much in Canada), wine storage ( I actually love the faint ceadar notes it left on my Yiwu), cave storage ( ok actually unused sauna), pumidor ( too much of the top notes faded), and left storage ( dried up).

    This is why I’m a big fan of wrapped and sealed storage. It just makes the most sense for me and my experience with storage.

    With that being said I have had issues experimenting with hot box and sealed storage as you mentioned above.

    Humidity/mold started on just the edges of a few cakes in hotbox due to the risks I took and misunderstandings, failure of my hotbox set up. I attempted to heat Saran wrapped cakes/ tongs in there and add humidity which I found was a risk bra cause there were different areas of different humidity without the ability of the humidity to breathe or move freely through them.

    Mylar bags- you saved me here (many many many thanks) reuse the ziplocks in your kitchen not for tea storage you need real food safe Mylar. 5% fail vs maybe 70?% ziplock. Oh no 5% failure!

    Failure of instrumentation happened as well. I always have a distrust of machine anyways.

    Overall puerh is resilient and none of my cakes have suffered the same issues- just a few at a time.

    However the stacks of unsightly Mylar in my house has become a completely different issue all together….


    Much peace

    • mgualt
      March 26, 2020 @ 11:09 am

      Hi Matt, that makes a lot of sense. It’s important that we share the pitfalls as well as the successes. I explored a lot of options like you did before starting the storage experiment. For the Mylar bags, I find that if they aren’t opened and closed too many times, they last a long time, at least 3-4 years in my case. It’s only the ones from my most frequent teas that got destroyed over time. But you can always reuse these as long as you heat seal them. Cheers

  6. kruchre
    March 26, 2020 @ 3:26 pm

    Thanks a lot for the update on the hot box experiment. Always exciting findings.

    I am not fully sure if I have understood the reason why you are proposing to remove the Bovedas from the box completely. Is the following reasoning correct?

    You humidified the cakes at room temperature with 69% Bovedas (which is relative humidity). So the cake maintains the absolute humidity at room temperature if it is sealed. When heating up the cake in the box, the absolute humidity stays constant due to sealing. But the relative humidity decreases at 32C. A Boveda pack would emit additional humidity at 32C to reach the relative humidity of 69% again. So absolute humidity would increase at 32C which rises the risk of mold. Is this the reason why you removed the 69% Boveda?

    • mgualt
      March 26, 2020 @ 4:04 pm

      No, the relative humidity does not decrease at 32C, because the bag is not simply filled with air. The tea cake has humidity and adds humidity to the air in the bag. See my first post on the subject. The Boveda pack is unnecessary in the long term storage, and I have found that it can lead to condensation and mold if there are irregularities in the heating. It is an unnecessary risk. I am not saying that it is guaranteed to cause mold. I am saying that it is an unnecessary risk from observations that I have made.

      Also, it is not that I am “proposing to remove the Bovedas from the box” — I never proposed adding them. The results that I have been reporting on for the past few years do not include Bovedas in the hotbox.

      • kruchre
        March 27, 2020 @ 2:41 am

        I fully agree that Bovedas are not needed in sealed storage if the cake is properly saturated. But I like draw some conclusions for myself as I am using more “traditional” pumidors (insulated boxes with 69% Bovedas at room temperature). I am always afraid in summer when temperatures go higher than >25C because I assume that Bovedas are only controlling the relative humidity and not the absolute humidity. The hygrometer inside the box shows more or less the same RH even at higher temperatures, i.e. absolute humidity must increase at higher temperatures due to the Boveda emitting more humidity to maintain a constant RH.
        Absolute humidity is the absolute water content in a certain volume (in g/ml^3). However, RH is linked to a certain temperature (i.e. RH varies over temperature for a constant water content). If you seal the cake at room temperature (e.g. 21C) with 69% RH and then heat it up to 32C the relative humidity inside the sealed mylar bag will decrease due to higher temperature. But the absolute humidity stays constant in sealed storage. The absolute water content is relevant if mold grows or not. So it might be more safe to remove the Bovedas even from “traditional” pumidors at room temperatures when the temperatures rise during the summer months. That is my conclusion from your article.

        • mgualt
          March 27, 2020 @ 10:05 am

          Thanks for your reply. There are two errors in what you say, let me explain.

          If you seal the cake at room temperature (e.g. 21C) with 69% RH and then heat it up to 32C the relative humidity inside the sealed mylar bag will decrease due to higher temperature.

          This is not true. Look at my first article about conditioning. The cake itself can affect the RH inside the container and it will actually rise at higher temperature.

          The absolute water content is relevant if mold grows or not

          This is not true, it is the “water activity” or generated relative humidity which is important for determining mold growth.

          But if I ignore these two steps, what about your final conclusion? I don’t know. If I were using a fridge pumidor, what I would try to figure out is how quickly does the fridge lose humidity. This can be determined by measuring some of your cakes generated RH periodically as you leave them in unhumidified minifridge. If it is very slow loss, you might be able to rehumidify once every year or maybe once very few months. This would be one approach… Periodic rehumidification.

          • kruchre
            March 27, 2020 @ 11:07 am

            Thanks for the explanation. Of course I have seen your conditioning experiment. The rise in RH for higher temperature was observed for cakes which have arrived freshly from Taiwan, or? Perhaps they might emit higher humidity at higher temperature as they have not been stable and conditioned to the local environment? But in general shouldn‘t it be more like it is described in the following app note?

            When taking a look on figure 2, the green curve shows a higher water content for higher temperatures at a constant RH. Therefore, if one has 65% RH at 20C one should not see a increasing RH in a sealed and fully stable environment as no additional is water content is added. I guess that Boveda packs change that behavior.

            Sorry to bother you. But I think your article gave me good food for thought. I guess it is more safe to remove the Bovedas in my semi-sealed pumidors for higher temperatures during the summer months.

          • mgualt
            March 27, 2020 @ 11:23 am

            Perhaps they might emit higher humidity at higher temperature as they have not been stable and conditioned to the local environment? But in general shouldn‘t it be more like it is described in the following app note?

            one should not see a increasing RH in a sealed and fully stable environment as no additional is water content is added.

            No! Both these assumptions are wrong. You should think of the puerh cake itself as a kind of variable boveda pack, where the rated RH increases with temperature. The cake will contribute more humidity to the air in the container when the temperature is increased. Most of the moisture in the cake is not available to evaporat.

            Again, the cake is a reservoir for water, it is trapped and can only escape slowly. It escapes more quickly at a higher temperature, and so when it reaches equilibrium in the container, the RH will be higher at higher temperatures. By the same token, if the container is not air tight, it will dry out faster in hotter temperatures.

          • Tertti
            March 9, 2021 @ 8:53 am

            I think your setup works and has little risks which are the most important things. But my guess is that your understanding of relative and absolute humidity in a closed environment are a bit faulty.

            Let’s say you condition one cake in 21C with 69%RH and it will become “fully conditioned” = it has sucked all the water inside it wants.

            Then if you put the cake to a closed environment with has air with 40% (a made up number) RH% and 32C. This would cause the RH% to DECREASE IN THE CAKE but it would INCREASE IT IN THE ENVIRONMENT (from the previous 40%). So I think you got the RH% in the cake and in the environment mixed up.

            And what matters most is the RH% in the cake.

            Please tell me where I’m wrong and how?
            Before that please check this calculator: https://planetcalc.com/2167/
            keep the absolute RH constant and change the temperature and you will see that if the temperature increases RH% goes down and the other way around too.

            Have you measured the RH% in the hotbox after adding some conditioned cakes? I would be interested in knowing how much is it.

          • mgualt
            March 9, 2021 @ 10:15 am

            Maybe you should read my posts about conditioning before you make any more comments. The part of your message where you are screaming in bold text is pretty funny in this context.

            The quantity that I can and do measure, besides the temperature, is what I refer to as “generated relative humidity”, which is, to a very good approximation., the “water activity” of the tea. This is one of the few things that can be easily measured, and my tests show that it is very helpful in controlling aging in storage. As you say (and as you can read in my 2017 post) the water activity will change at different temperatures. However the point is that the tea can be conditioned at any temperature if the right GRH is chosen. Added humidity is more risky at higher temp for practical reasons, and so I do it at a lower temp.

            Once you have a way of measuring water activity or GRH, you can calibrate it by comparing to well stored cakes. That is why the earlier tests were important.

          • Tertti
            March 9, 2021 @ 10:39 am

            Thanks for the reply. Didn’t mean to convey screaming. Sorry about that.

            Thanks for the link to the old article. Indeed the findings are the exact opposite what I was writing. It is very interesting.

            Do you know why your findings in the don’t follow the physics principle “temp goes up RH goes down in a closed environment”. I very much would like to understand how my thinking is wrong.

            I didn’t really understand the GRH and how to actually measure it. I understood from your explanation kind of what it is but not how to measure it.

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  8. kruchre
    March 27, 2020 @ 12:37 pm

    Alright, this context works for me. I guess I understood. By implication my cakes might contain much more water than they can emit at room temperature. Even if they have been conditioned with Bovedas for years. Haven’t seen it that way before. Thanks a lot.

  9. Daniel
    April 14, 2020 @ 4:48 am

    I really like the hotbox, thank you for sharing your experiences. I am thinking about building one too inside a mini fridge. In case of safety of the setup, you mentioned a power outage. In that case the temperature inside the fridge/cooler would drop slowly and the cakes would have time to reabsorb the otherwise increasing humidity right? Also when accessing a cake inside the storage, could the incoming cooler air cause some condensation? Or doesn’t it matter when closing it back up fast enough?
    Greetings from Germany,

    • mgualt
      April 14, 2020 @ 10:59 am

      Power outage and briefly opening don’t cause condensation… Remember that there is no extra humidity and RH is kept around mid-low 60s. Temp does not drop that fast inside the bags, but even if it did it still wouldn’t get to 100rh in the range we’re discussing.

  10. Tom A (aka tea.gull)
    April 18, 2020 @ 10:57 am

    This certainly generated a lot of discussion!

    I’m taking the risk of keeping 65% Bovedas in the mylar, because I observed that even using room temperature conditioning for extended periods with a 69% Boveda, my cakes didn’t hold the desired relative humidity in the hotbox at 32C. If the power should go out for an extended period and I’m not around, I do risk significant mold development. Considering our current circumstances of isolation, I’m not too concerned.

    I have had some light mold develop, primarily on cakes at the outside of the cooler, when the temperature probe was set in the middle of the box. Oops. Interestingly, the cakes that developed mold were the poorest quality teas in my collection, higher quality teas seemed to resist mold development under the same conditions.

    Additionally, after a year and a half, my oldest teas, that suffered almost a decade of natural storage in the northeast, are struggling to continue to developing at this point, whereas my younger and mid-aged teas that seemed to still have some life left are now quite active and transforming nicely. I’m wondering if the long, dry storage of the older cakes fully killed off the natural flora, but there was still enough left on the younger cakes to be re-activated in the hotbox.

    • mgualt
      April 18, 2020 @ 12:52 pm

      It seems to me that you yourself proved that it’s a strong mold risk, as you actually had mold development. Once the cakes reach humidity take the bovedas out, then it should be safe. I would definitely not assume that some kind of “flora” are dead and so the cakes can’t age, just rehumidify the cakes. There is plenty of enzymes in the tea itself to help oxidation. “Flora” is not necessary relevant. Best wishes.


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