Making rubber molds for concrete or hypertufa castings is ridiculously easy.
It’s also one of those things that seem scary…until you’ve done it!
You can start small- try a rock, or an apple or orange. If you use garden produce it must be fairly solid and fresh-no softer than a firm orange. It will be sitting in the molding compound for several days and will start to rot otherwise. Don’t plan on eating anything after molding it either.
You’ll need:
- Brushable latex rubber- or other brush-on molding compound. I use Douglas & Sturgess Latex 74 (love it!)
- Mold release- this is so your finished mold comes free of your vegetable, rock or whatever you are molding. If none is available, use cooking oil. Seriously. You can brush it on and it works beautifully. I even used Bakers Joy spray on a demo piece here and it worked-but only for a casting or two. It seems to tear up the mold material very quickly.
- Throwaway paint brushes. I buy the cheap dollar brushes from the superstore. They are no good for anything else after being used with the latex so plan to toss them.
- Plastic wrap or plastic grocery bag to wrap your brush between uses.
- Gloves – required if you have a latex allergy and are using latex molding compound.
- A place to do this- make sure you have good airflow, please
- A work surface. I love the foam meat/produce trays for most smaller items. The molding compound doesn’t stick to them! A plastic trash bag works nicely too but may bunch up beneath larger items.
Getting started
Clean off and dry whatever you want to mold. Think about how you are going to un-mold it, too. Really weird shapes do not always want to come out of the mold as easily as they went in.
We’ll use this little dried warty gourd for our demo. It has lumps and bumps, but no undercut areas that will cause the mold NOT to release.
If it has a stem either remove it, or if it’s a big stem (like on a pumpkin or gourd) smooth it and seal any weird rough areas that might cause problems with your mold. You do NOT want to have any deep ‘undercuts’ that will catch and tear or snag your mold. That might ruin your casting later and then you would cry. If you have any odd spots, fill them in with caulk or putty and let it dry before continuing.
Now lightly cover (brush or spray) the entire item with your oil. Make sure the item is covered well, but not dripping or runny.
I used a *baking spray inside this little gourd then took a small paint brush and worked the spray around the bumps and into the crevasses. Clean up the excess! I even wiped up the excess on the tray. I’m so tidy.
Now we’ll start brushing on the molding compound
When using the brush-on stuff it’s best to do a thinner first coat and brush it all in one direction. Once that had dried you’ll do a thin second coat, in the opposite direction. So first coat- up and down, second coat- back and forth, etc. but feel free to reverse that order.
Depending on the size of the item and manufacturer’s directions you may need a number of coats. With the Douglas & Sturgess Latex 74 I generally apply four coats; maybe five on larger items that are oddly shaped.
Remember, your piece has to sit and you have to remove the item from the finished mold somehow, so don’t completely cover the base.
Now wrap up your brush in plastic and close the container while this first layer dries. On a warm sunny day with low humidity I can often re-coat in about 5 hours. In really wet, cool or humid weather it may take a lot longer.
Here is coat #1
It dries and looks MUCH thinner and you will be able to see thin or even bare spots you missed. Just make sure each successive coat goes on it covers completely and dries before adding the next. Also- most first coats aren’t as gloppy looking as this. Those warts really added texture.
When all your coats are completed and dried it’s time to un-mold your piece!
Let’s remove the mold from this Jarrahdale pumpkin.
Carefully remove your item by peeling back the rubber. You may need to trim the opening a little. That’s OK, you can trim it later so the finished cast will sit straight.
You can see how I painted this stem. It was cracked and needed to be sealed before I started molding this pumpkin. Any paint that comes off with the mold can usually be brushed off thanks to mold release!
Next I will trim the ragged edges on the opening of the mold and it’s ready for casting!
By the way, this is exactly how the mold will look when you cast the concrete or hypertufa into it.
* * Something in the Baking Spray I used on the little demo gourd wreaked havoc on the latex mold. If you have nothing else available and only want one or maybe two castings go ahead and use it. If you want to use your mold more than once either use vegetable oil and a brush for best results. I’m also rethinking ever using baking spray again in the kitchen.
Making your own concrete garden art is SO much fun. Next, this time we’ll do some casting!
Before you get started there are some things you will want to have on hand:
- a place to work- with above freezing temperatures for several days to a week
- concrete
- water
- tub or bucket for mixing concrete
- tools for mixing concrete
- mold release, or oil and old paint brush
- sand -used or old sand from a child’s sand box is fine
- tub for holding sand -the sand filled tub serves as mold support while pouring and curing concrete
You can use hypertufa, concrete, or a mortar mix. When casting smaller pieces or items with detail I use a mix that has little to no aggregate, which in plain speak are rocks or pebbles. A sand concrete or mortar mix works well and you can usually buy a bag at your local lumber yard or home store. It’s not expensive, but it is heavy!
Ready? Let’s get started
Brush or spray your mold release into your mold. Make sure all parts of covered or the concrete will not release!
Place some sand in your tub. This is a type of “back up” and will hold your mold in place.
Now wet some of the sand and gently push it up against the sides of your mold. Only go about halfway up for now. Try not to get sand inside the mold if you can help it.
Mix up your concrete.
Add a little bit of water in the tub or container and add the concrete mix. Mix well. You will want it to be like a very thick batter so depending on what sort of mix you are using you may have to adjust the water and dry concrete amounts to reach the desired consistency.
If it seems a little thin, don’t worry too much. It will cure eventually-it just may take a day or two longer before you can remove the mold. You will also know how thick it needs to be next time!
Pour concrete into your mold about halfway up, or up to the sand support.
You may need to push up more wet sand around the mold to support it as you continue pouring.
It you are working on a larger piece you may want someone to give you a hand with this.
You may discover you have too much or too little concrete to finish the mold.
I’ll definitely need more concrete to finish this pumpkin.
If you need to mix up more concrete do it quickly and pour it on top of what you have already poured. Fill to nearly the top of the mold and jostle the sand tub a little. This helps the concrete to settle into the mold.
Extra concrete can be poured into old plastic tubs, plastic bowls or jello molds that have been treated with mold release. You can also make sand castings by hollowing out wet sand and pouring the concrete right in. The finished product has a sandy finish- nice for stepping stones in slippery areas.
Before concrete sticks to all your tools and bucket, go rinse everything off!
Next you’ll want to try to remove any big air bubbles in your concrete. To do this you can use a stick -I’m using a wood skewer -and gently move it through the concrete to release any bubbles. Do not worry if you see water rising to the top of the mold. It will evaporate soon.
Lightly cover and leave the mold ALONE for at least 48 hours, although 72 hours is better for thick pieces like these.
If you have a really large heavy piece like the pumpkin below, let it sit for four or five days. Tempting as it is, if you remove the mold too soon the concrete will crack and break apart.
Do you see the dark spot at the top of the concrete? I nicked it with a fingernail at about 16 hours of cure time. The concrete is very soft at this point and will take several more days of curing before I’ll try to remove it from the mold. That’s another thing…concrete doesn’t actually dry. It cures, and the longer and slower, the better!
OK, so now you have chewed your fingernails to the nubs in anticipation of the finished concrete cast. Well let’s take a peek !
This little gourd has been curing for over 48 hours, so let’s get him out of that mold.
Peel the mold back gently. If the concrete seems crumbly or feels like it may break, set it down and let it cure for another day or two. This gourd could probably use another day of cure time, but I’m removing him so we can have photos.
And here’s a view of the top. Yep, the tip of the stem did not release properly and broke off because I should have waited another day. Darn it.
At this point the concrete is not thoroughly cured and is still workable. I’ll gently brush off the rough spots and carefully scrape away any rough edges. Then this little guy will sit for a few more days to cure completely. After that he’ll be ready to come out and play in the garden!
In the hot summer months I set new concrete pieces in the shade and hose them down every day until they are fully cured- which usually takes about a week. In the Spring and Fall I keep them in the greenhouse under a bench and make sure they don’t dry out.
Remember that I mentioned using Baker’s Joy as a mold release? Well here is a photo of the mold after using the Bakers Joy spray in the mold twice. Obviously I can’t recommend using it for repeated uses…mostly because your mold will disintegrate! Regular pump sprays with nothing but vegetable or Olive oil will work nicely.
You can also use this same mold/casting method for other garden art pieces
…including concrete leaves for birdbaths. This method takes more time and materials, but it’s great for creating items in the winter when giant leaves aren’t available!
We first cast a real leaf in plaster. We then make a concrete ‘backup’ which protects the fragile plaster. A rubber mold of the plaster leaf comes next. Once dry the rubber leaves can be cast the same way you would a fresh leaf!
A final note- concrete does not produce a smooth finish when cast so some of the detail will be lost. Hypertufa is even more coarse than concrete, but has a great ‘old’ look to it. If you want finer detail on your finished casting go with a mortar mix- it produces a smoother finish. Happy Casting!
If you make some casts of your own I would love to see your creations. Better yet, share your photos on our Facebook page!
Source: Gardening 101 com