Having completed my first attempts, or as I like to call them, experiments with concrete, I decided that I really needed to find a more versatile way of creating the molds for the rectangular pots. While doing some research, I found this video by ReoFlex which covers how to make a one piece urethane block mold. The mold that is shown in the video intrigued me and I wondered if I could make something like that work with concrete. I drew up some plans and began to build it but it would take me at least a week or two to complete it. In the interim, I carried out further experiments.
On my wish list was the ability to create items with thin walls – 1/8 – 1/4 inches. My first shot at this was done using two plastic containers from the deli section of our local grocery store. I made a fluid mix using the Quikrete Non Shrink Precision Grout and poured it into one of the containers. Then I pressed the other one in and placed some weights into it to keep it from floating up. I used small foam blocks between the two rims of the containers to get a ‘fairly‘ even wall spacing. Here is the result:
I was amazed at how smooth the surface was and at how few air bubbles there were. I learned later that this is because of the very smooth surface of the container and the fluid mix. I was not that impressed with the top edge or the consistency of the wall thickness. I had to use a file to grind down the top edge and again realized that I needed a better way to smooth things out.
I decided to try something larger using the countertop mix and an aluminum bundt cake mold. As the countertop mix can not be made as fluid as the grout, I had to press the mix into the mold. Here is the result:
If you try this, us a LOT of release agent, I basically destroyed the bundt cake mold removing the pot from it. Fortunately, it was a used one that only cost me a dollar. Note all the air bubbles. This piece is quite heavy and will get used in our garden somewhere.
A thinner mix
The budnt cake mold experiment convinced me that for what I was trying to accomplish, a more pourable (fluid) mix was needed. I did more research and found a local company called National Concrete Accessories. I gave them a call and spoke to a young man named Ryan who was extremely helpful and patient with all of my questions. He suggested I try a countertop mix made by a company in Vancouver called Starpatch Concrete Products. I contacted Starpatch via e-mail and another very helpful man, Bendix, suggested that I try a product called Kast Krete. I had a look at the data sheet … wow! Kast Krete is a specially formulated polymer-modified concrete mix. It can me mixed to a consistency like pancake batter which makes it very easy to pour. It’s white and can be removed from molds in as little as 15-30 minutes. I immediately ordered 2 bags from Ryan as well as a bag of their Counter Mix and picked it all up a few days later.
I decided to start with a small pot and I wanted it to have a drainage hole. This time, I used two Ikea food containers that nested well together. I drilled holes in the bottom center of each container and then ran a screw through the holes and placed a 1/8″ spacer made from some rubber tubing between the 2 containers. I then tightened that up with a wing nut. This gave me a more even wall thickness. Alas, I do not have a picture of that to show you but I drew up this diagram to better illustrate the pot mold.
I mixed my first batch of Kast Krete and then poured it between the 2 containers. I jiggled it around so that the mix was well distributed between them. Here is the result.
Now we’re getting somewhere! Note the lovely color, the very smooth surface and the drainage hole. And, this came out of the mold in one hour!
The top edge was still pretty ragged so I tried sanding it using a 60-grit sandpaper. I just placed the sandpaper on a board, then holding the pot upside down, I made circular motions and that worked very well to remove the jagged edges.
Pleased with the results of that pour, I made another pot using larger containers and a thicker wall. I added some charcoal iron oxide to the mix – my first color experiment. Here is the result of that pour:
Note that the top edge is getting smoother. Again, I used the 60-grit sandpaper to knock off the jagged edges. Then, I used these!
I found these on ebay from Caliber Stone Tools. They are foam-backed straight diamond hand pads for use on granite, marble, concrete, glass edges and surfaces. The set has 7 different color-coded grits, from rough grinding to final honing: 60, 120, 220, 400, 600, 1800, 3500 grit. For the pot above, I have only used the 60 and 120 thus far. These pads are great! I’ll re-visit this pot soon with the finer pads.
What I learned and what I want to learn
- I can now smoothly sand edges. I need to learn more here but I am well on my way.
- Still – I would LOVE to have a very smooth edge without sanding. In the next post, I will show you what I discovered about upside-down vs right-side-up!
- Kast Krete is an amazing product. I just ordered two more bags. Another white, and a tan. Looking forward to trying the tan.
- Vibrating/tapping/jiggling the mix in the mold is very important if you want a smooth surface and no air bubbles. Here is a pot I made with the Starpatch counter mix that I did not spend enough time getting the air out. It was made using the same mold I used in part 1 – the grey pot. This one has 1/2″ thick walls though. I just made my foam insert 1″ large in all dimensions.
In closing this post, I just want to again thank Ryan at National Concrete Accessories and Bendix and his brother Peter at Starpatch Concrete Products. All three of them have been very helpful and happy to share their knowledge with me even though I am a very small customer in the grand scheme.
See you in part 3 where I will show you the evolution of my adjustable mold!
Read more in this series:
Diamond Hand Polishing Pads – Caliber Stone Tools
All other photos by Gil Namur – All Rights Reserved