Introduction: Concrete Trivet
I've always wanted to try my hand at making something from concrete, so I figured a trivet (a fancy hotpad) for the kitchen would be a good place to start. It took more than a few tries to get it right, so I wanted to share some tips and tricks so that you can take your own ideas and turn them into something "concrete" (pun intended).
Precision Grout (~$20 for a bag, you could probably make 30+ trivets from one bag)
Silicone (~$40 for a kit, my mold used up about 1/2 of the kit)
Automotive spray primer (~$6, used less than 1/4)
Normal spray primer (~$5, used less than 1/4)
Mold release (~$12, you'll use a little for each trivet you make)
Some medium-high grit sandpaper
Plastic cups/spoons for mixing the concrete
3D Printer (or some other method of creating the pattern)
Step 1: The Pattern
First, we'll need a pattern of the trivet we want to make. The pattern should represent the final form of the desired part, including surface finish. I used a *3D printer to print the pattern (STL file attached below) and then used the following process to get a clean, smooth surface finish (see photos):
Step 1: coat the pattern with automotive spray filler (this is thicker than normal primer and does a great job of filling in cracks/layer lines quickly).
Step 2: sand with medium to high-grit sandpaper as needed (ideally, each successive layer should require less and less sanding).
Step 3: thoroughly clean off the dust from sanding and re-coat the surface with the automotive filler. Repeat steps 1-3 until the desired surface finish is achieved.
Step 4: once there are no more layer lines, paint the pattern with one final coat of standard spray primer. This provides the best possible surface finish so as to keep the silicone from adhering to the pattern.
I borrowed aspects of this method from juresnip, who has a great Youtube video on the subject.
*If you don't have access to a 3D printer, get creative with how you make your pattern. It can be made from wood or other materials, as long as it is coated well with primer to get the surface finish you're looking for.
Step 2: The Mold
Now that the pattern is complete, it's time to make the mold!
First, we'll need to add a base and walls to our pattern to enclose the silicone. I had some sheets of craft plastic that worked perfectly for this, but you could easily use cardboard or another material. When constructing the walls, be sure to leave enough space between the pattern and the walls so that your final mold will be robust. For me, that was about 1/4".
Using double-sided tape, attach the pattern to the base, keeping it as centered as possible. Of course, any other adhesive or glue could be used for this step but the tape worked perfectly for me and made for easy removal afterwards.
Next, mix the silicone according to the instructions provided (in my case, equal parts of A and B). Be sure to mix thoroughly, I can't emphasize that enough. Also, if you're not sure how much you need, Fusion360 and other modelling tools will often have a "volume" feature that tells you how much volume is contained by a given 3D model. Remember, you want to know the amount of volume between the walls/base and the pattern, so keep that in mind. I ended up mixing a little extra just to be safe, so I would suggest you do the same.
Before pouring, you'll want to spray some mold release onto the pattern and walls/base to ensure easy removal once the silicone is cured. Be sure to get the right mold release, as some are not suitable for silicone molds and may actually cause the silicone to adhere to the surface of your pattern.
When pouring the silicone, be sure to pour from high up (8-12 inches above the mold). I heard this tip from multiple sources and it works like a charm at reducing the amount of bubbles in your final mold.
Now the easy part! Wait for 24 hours to let the silicone cure, then remove your base/walls and the pattern itself (the moment of truth!). Remove the pattern slowly and be careful not to tear the mold in the process.
Step 3: The Concrete
Now the fun (and somewhat messy) part! This may take a little bit of experimenting but now that you have a mold, you can make as many trivets as you'd like.
First, spray the mold with mold-release. This will help extend the life of your mold and make it easier to remove your trivet.
Next, mix ~1/2 cup of water with 1 1/2-2 cups of precision grout (I acknowledge that precision grout isn't technically concrete but for some reason "Grout Trivet" didn't go over well with marketing). Add the grout slowly as you mix, making sure that no clumps form. What you're going for is a sort of pancake batter consistency where the concrete can flow but isn't too runny. Once again, this may take a bit of trial and error but that's the fun of doing it yourself.
I achieved the most success (and least amount of air bubbles) by pouring in one spot of the mold and letting it "flow" into the details of the mold. It is helpful to place the mold on a portable, flat surface so that you can shake the mold lightly from side to side. This helps remove air bubbles and ensure the grout has made it into all of the cracks and crevices.
Once the mold is filled with grout and the top is relatively level, leave it for 8-10 hours to set. While the grout is curing, it's important that it doesn't dry too quickly. I ruined the first few trivets by leaving them outside in 90 degree weather (they become crumbly and weak because the water evaporates too quickly, resulting in a weak bond). To avoid this, bring your molds inside where than can cure in a more temperature-controlled environment.
After 8-10 hours, use the fingernail trick to see if they are ready to be removed from the mold. Press lightly against the grout with a fingernail--if it leaves a mark, it's not ready to be removed. If it doesn't, carefully remove the trivet from the mold and let it cure for another 12-24 hours.
To remove some of the uneven coloring of the grout, use some medium or high grit sandpaper to smooth out the surfaces and produce a more uniform appearance (see pictures).
Step 4: The Pad
We're almost there! The last step is to add the cork base so that the trivet won't leave any marks or scratches on the surfaces it's protecting. I bought a roll of 2mm cork sheet from Amazon and it worked like a charm.
Trace the outer edge of the trivet onto the cork sheet, then cut 1/4-3/8 in from that line. If you like the look of the cork and have some thicker material on hand, it might look cool to have the cork right up against the edge of the trivet so that you can see it from the sides.
Epoxy is probably the ideal solution for adhering the pad to the concrete, but the gel super glue that I had on hand did just fine as well. I've used these trivets for months now and the pads aren't showing any signs of separation.
There you have it! Your very own, custom concrete trivets. They make great gifts for friends and family, and have held up to quite a bit of abuse. Would love to see pictures if you try making some yourself!
If you're interested in following along on this or future projects, consider following me on Instagram at @radial.design
Second Prize in the
Stone, Concrete, Cement Challenge