Types of plastic wrap for pots to slow or increase drying speed.

[Min]

Recently I started using pieces of a painters plastic drop sheet to cover pots as I had misplaced the roll of a different brand of plastic sheeting I had been using for years. There is a marked difference in the drying rate between the two types of plastic. It got me wondering about why this was happening.

Turns out there are at least 3 types of plastic sheeting that are broadly available and they have measurable differences in water vapour permeability.  Low density polyethylene (LDPE), polyvinyl chloride (PVC) and polyvinylidene chloride (PVdC). I don't know what the original plastic sheeting I was using was made from or the brand, it's a large roll that I've been using for many years.  The painters plastic drop sheet I just started using is this one. My hunch is that it's the type of plastic that is making the noticeable difference. Going forward I'm going to experiment further, pots I want to dry slower will get the drop sheet plastic and for ones I want to dry quicker my original roll of plastic. I should add all the pots were covered in my usual manner. 

[Dick White]

Many studios use plastic dry cleaner bags to wrap the fresh greenware. I have not been in a dry cleaner establishment in the 15 years since I retired from my real day job downtown, so I ask students to bring in theirs. One studio I work with went to whatever supply house and got a full 1000 piece roll just like the cleaners use; we've been working that roll for 4 years and still have half. The dry cleaner bags seem to have a reasonably predictable permeability for drying by next class time, and they are thin, light, and flexible so the ware is not smushed when wrapping it. The problem is that few students take the time to cut the full bag into appropriate size pieces, so the work-in-progress ware shelves are loaded with single mugs on 12" square boards wrapped in an entire bag (and because it is still a bag, it's a double layer draped over the mug). And then some Helpful Hanna/Harry brings in an armful of garment bags scavenged from the department store when the stock clerk was unwrapping and setting out the spring fashions - a completely different type and weight of plastic. And don't get me started on the empty clay bags that show up in the bin. After all, plastic is plastic, isn't it? Well, no, as you point out there is plastic and there is *plastic*, not all the same.

All that said, when the bin is running low (with 125 students cranking out production), I have picked up a package of cheap lightweight painter's drop cloth from the local blue or orange big box, like your suggestion (except I get the lightweight 3-pack, yours is midweight). I've also noticed that the community center uses the cheapest lightest thinnest trash bags, so I have raided the janitor closet.

Carry on, we are comrades in arms...

[Roberta12]

1 hour ago, Min said:

Recently I started using pieces of a painters plastic drop sheet to cover pots as I had misplaced the roll of a different brand of plastic sheeting I had been using for years. There is a marked difference in the drying rate between the two types of plastic. It got me wondering about why this was happening.

Turns out there are at least 3 types of plastic sheeting that are broadly available and they have measurable differences in water vapour permeability.  Low density polyethylene (LDPE), polyvinyl chloride (PVC) and polyvinylidene chloride (PVdC). I don't know what the original plastic sheeting I was using was made from or the brand, it's a large roll that I've been using for many years.  The painters plastic drop sheet I just started using is this one. My hunch is that it's the type of plastic that is making the noticeable difference. Going forward I'm going to experiment further, pots I want to dry slower will get the drop sheet plastic and for ones I want to dry quicker my original roll of plastic. I should add all the pots were covered in my usual manner. 

Who knew???  Certainly not me!  I know that some plastic is thinner, heavier, but that is the extent of my knowledge!  Thanks for the research @Min

[Callie Beller Diesel]

Makes sense. We think of plastic as totally impermeable, but it isn’t entirely.

With the recent addition of a cat to my life, I’m thinking of building a damp box out of one of my shelves, so she’s not climbing through wet pots to get to her favourite studio perch. I have some vapour barrier that I was going to use, which will be a change from my entirely too old stash of dry cleaner plastic. The vapour barrier I have is only 3 mil I think, so it might be a closer comparison than a heavier 6 mil.

[Kelly in AK]

Yeah, the different types of plastic have different “permeability” ratings. Even though they’re all waterproof.  Fun fact, Mylar is as close to zero as you can get which is one reason they make balloons out it.

I’ve certainly noticed it with clay I’ve bagged up for storage.