It's summertime-- time for picnics and camping and frozen treats! Only these things don't tend to go together very well... that pesky summer heat makes it tricky to transport frozen goodies very far from a freezer. And no one wants to find sad, melty ice cream swimming in their cooler full of ice. A couple of years ago I was spurred to come up with a solution for this (I wanted to serve a granita appetizer for an outdoor food expo in August). I thought if I froze the granita into a solid block of ice, it would work better than either regular ice cubes, or trying to hassle with dry ice. It worked like a charm! After a few hours of scooping granitas, my frozen blocks were still perfectly frozen solid and protecting my precious granita. But my method was a little hairy (it involved very particular containers, and knots and weights.) I knew that there must be a better way to pull the same concept off, and vowed that I'd return to it later. Happily, the lightbulb went off this year, and I came up with a nifty trick to pull the same thing off with simple tools, and very quick set up. All together, this cooler takes about half an hour of active time to make. And once you've got your containers and method all worked out, you'll be able to set up one of these guys to freeze in just a few minutes.

I also made a fancy macramé carrier to make transporting my cooler easier and more comfortable. (And it can be reused indefinitely for future frozen treats.) Once filled with the ice cream, sorbet or granita of your choice, you can carry this guy around for about an hour outside of a cooler. Or you can throw the whole frozen block into your cooler, and it will act as your cold-pack, keeping the rest of the food in your cooler refrigerated, and then you can break into the contents of your freezer for dessert. There are few things quite as decadent as whipping out a magically frozen treat in the middle of your summer picnic or hike. Enjoy!

Step 1: Strategy

The difficulty with making a vessel out of ice it that you need an interior container (where your frozen goods will be stored) to be suspended exactly in the center of an outside container while that outside container is filled with water. So if the interior container is empty, once you start to fill the exterior, that little guy is going to want to float. On my first experiments, I used a combination of weights and twine to tie the container in the right place. That worked, but it was rather tricky, and time consuming. I was playing around with different weights and figured out that if I filled the interior container with the right amount of water, I could get it to float at exactly the right height I needed. But if I would have frozen the whole thing solid, getting that center piece of ice out would be next to impossible. Then it hit me-- salt! If I made a briny salt solution in the center (which has a much lower freezing temperature than fresh water), the outside would freeze solid, and the inside would be liquid (or at least slushy) enough to pour right out. Science! After filling the center with salty water, all I needed was a few pieces of masking tape to secure the interior container in the center, and my vessel was ready to freeze. Once the outside is frozen solid, you'll just pour out the salty center, rinse out the interior container, and unmold your new ice vessel.

Step 2: Materials

2 plastic containers 1 in the size you want the inside of your final vessel to be, 1 at least 1" larger in all dimensions. salt (inexpensive kosher salt does the trick)

masking tape or other sturdy tape that will still hold if slightly damp.

water

liquid measuring cups

20-30 feet of twine or other sturdy cord (if you want to make the optional holder for the cooler)

a 1" (or smaller) diameter plastic lid, the kind you'd get from a water bottle works well.

6" of 1/2" rigid plastic or metal tubing (if you want to make a handle for your holder )

Tools:

scissors or utility knife

kitchen shears or a sturdy paring knife

freezer with enough level storage space to freeze your molds

pliers

Select the right containers The first big decision is how much volume you want your finished vessel to hold. (Remembering, of course, that this ice container works best for one-time uses, so it will help to be precise about exactly how much frozen goodness you will be eating/serving.) A quart or a pint is probably a good size to start with. I used the standard quart containers that you're likely to find in any restaurant. Once you've decided on a quantity, start looking for plastic containers that are relatively compact in shape that hold the volume you want. The more compact a shape, the more efficient your cooler will be. Less surface area/volume ratio means less places for heat to get in to your freezer. Found a good plastic container to shape the inside of your vessel? Great! Now you'll need to find second plastic container that is at least 1" larger than your first in all dimensions. This container is going to make the outside shape of your cooler. At a hardware store, I found a nice 2 1/2" quart paint tub that is the right size to be the outside mold to my 1 qt container. (Since this container isn't actually going to come in contact with food, I wasn't too fussy about finding food-grade plastic. That said, I certainly wouldn't use anything that had been used to store anything you wouldn't want near your food.)

If you want to make a lid for your vessel, you'll also need to find plastic containers to shape the lid. If the sides of your exterior container are relatively straight (not tapered) you can use the same container to shape a lid. Figure that you'll want to make your lid around 2" deep. You'll also need to find a container that is the same diameter as the outside of the lid for your interior mold (you want to make an indentation in the lid that will fit around the top of your plastic container. Because the walls of my container were tapered, I used a different plastic container to mold the lid for my vessel.

If you've rounded up the containers you need, you've already finished the trickiest part of this whole thing. (Really!) Now we can move on to the fun part and make some ice.

Step 3: Mold and Freeze the Vessel

Make a salt solution- Fill your interior mold with water to 1/2" from the rim. Measure the amount of water that it takes to fill your container. Add 3T salt for every cup of water and stir to dissolve. Pour salt solution into interior mold. Place plastic lid (if you have one) on interior mold. Carefully transfer the interior mold into the exterior mold.

Fill mold. Slowly pour fresh water into the exterior mold, If your interior mold is uncovered, be careful not to splash onto the inside. Once the interior mold begins to float, slow down and keep pouring until there is at least 1" of water underneath the interior mold.

Secure the inside container in the center. Use a few pieces of tape to tack the interior mold right in the center. This doesn't have to be the most resilient set up-- just enough to keep the container from floating over to one side.

Freeze. Set your vessel mold in the freezer and leave it alone for at least 4 hours and up to 24. (I know you want to peek and play with the salt water, but it's really best to leave it alone to freeze solid for at least 4 hours.)

Step 4: Mold and Freeze the Lid

Make the handle. Cut a 6-8" piece of your twine or cord. Cut two small notches into opposite edges of your plastic water bottle lid. They should be just big enough to hold your piece of twine. Fit your piece of twine into the lid and invert. Tack the water bottle lid to the center of your exterior lid mold with two small pieces of tape. (Later, you're going to remove the plastic lid and the exposed twine will make a handle for your lid.)

Form and fill the mold for the lid. Use the same technique for shaping the lid as you did for the main vessel. Fill the interior mold with salty water, and carefully fill the exterior mold with fresh water. Tack the interior mold into the center, and transfer the whole thing to the freezer for a minimum of 4 hours.

Step 5: Unmold the Vessel and the Lid

Unmold the vessel. Depending on how long you left your vessel to freeze and how cold your freezer is, the outside piece of ice should be frozen solid while the inside will be liquid or slush. (If it looks solid, just poke at it with a fork and it will most likely easily be reduced to mush.) Pour or scoop out the brine in the center container. Leave your molded ice out to at room temperature for 5-10 minutes. (This is called tempering the ice. It reduces the chances of cracking your entire ice vessel when you start manipulating it.) Tug at the interior container and see if it is still frozen in place. If it comes out, great-- you're done! If not, pour room temperature (not hot!) water into the center and let it sit for a minute. Test again, and repeat until the plastic container slips out easily. Invert and remove the ice vessel from the outside container. Cool, right? Now you can rinse out your salty container and return it to its ice cozy. At this point, you can fill your cooler with ice cream or whatever other frozen treat you plan to store in your freezer. Return your vessel back to the freezer. You can choose to either remove your vessel from its outside container now, or to keep it in there for storage in the freezer.

Unmold the lid. Use the same technique as you did the vessel (only you don't need to return the plastic container to the lid.) Flip the lid over so that the handle is facing up. Use a sturdy knife or kitchen shears to carefully start picking away at the ice in the center of the water bottle lid. Working little by little will get you the best results here. Once you've got most of the ice removed from the plastic lid, pour a little room temperature water into the center to help melt the plastic free from your ice lid. It may take a little patience, but eventually you'll be able to remove the plastic lid and the masking tape. I found a pair of pliers was helpful in pulling the plastic lid loose. Once your lid is free, return it to the freezer. (I would recommend not setting it on top of the ice vessel at this point-- it might have the tendency to freeze the whole thing shut.

Step 6: Make a Macrame Holder (Optional)

It isn't absolutely necessary that you have a holder for your cooler (and you could certainly fashion one in any number of styles.) But carrying around a large piece of ice is rather slippery and cold-- so having something to help you get a grip on it definitely comes in handy. I wanted to make a straightforward, simple and functional holder that wouldn't require too much specialized equipment. I settled on a simple macrame design that can be made out of almost any sturdy type of twine or cord you have laying around. I promise, this is really simple to make, even if you haven't done any macrame or don't know any fancy knots. This design works best on a round vessel, but it could definitely be adapted to different shapes. I give specific measurements which are apropos to the size of vessel that I made. The same concept would work for a larger or smaller vessel, but the specific dimensions I site would have to be scaled up or down to fit your vessel. And you only need to make this sling once-- if you use the same containers to mold coolers in the future, you'll already have a sling ready to go!

Cut 4 lengths of twine. For my 6" diameter and 8" tall vessel, I used 5' long pieces of twine (the length you need will vary based on the size of your vessel and the thickness of your twine.)
Tie or thread handles. If you want to make handles out of tubing, cut two 3" pieces of tubing. Thread two pieces of twine through each handle and slide the handle to the center of the twine. If you are not using tubing, you can just tie two knots to secure space for a handle at 3" apart.Take two of the lengths of twine and tie a knot 8" from the edge of the handle. Repeat with the other handle. Take one of the remaining loose pieces of twine and tie it to another loose piece from the other handle at about 7 1/2" from the edge of the handle. Repeat on the other side. Now you have your first complete row of knots. If you have your exterior mold free, grab it and fit the top of your holder around the vessel. Your top row of knots should sit evenly if you spread out the the knots around the top edge of your mold.
Tie a second row of knots. Separate the two strands-- one will go to the right and one to the left. And two strands will come together for the next row of knots. Measure 3" down from your first knots and tie off your second row of knots. (The distance of twine between each set of knots should be 3".)
Tie third row of knots. Use the same procedure and tie a third row of knots. Once again, test out your sling on your mold and make sure that it fits. Tie base. Invert your container and adjust your knots so that the third row of knots will be on the bottom of your container. You now have four sets of twine arrayed around the base of your container. Tie each opposite pairs of twine together using a secure knot for joining two ends together (I suggest using a fisherman's knot-- see my images if you need a few pointers). I couldn't figure out how to get the knots on the base to look tidy, so I settled for functional, and knowing that they weren't going to be on display anyhow. Now you can test out your sling! Your frozen ice cooler should slip right in and be ready to go

Step 7: Serve

Now your mini freezer/cooler is ready to use! Remember that it will be more firmly frozen if you let your frozen treats sit for 24 hours in the freezer before you take them elsewhere.