Introduction: How to Make an Agar Topography
This is a method I developed during a collaborative project I made with Aman Tiwari called Soylent Saviour. We were interested in the local cultural dynamics of the Pittsburgh technology/startup scene, and wanted to explore how use of visualization and biological metaphor can create dialogue and reflection about the relationship between our tech community and the larger social ecology of Pittsburgh. We used physical visualization methods, robotics, and biological specimens to create a unique experience of data describing social change in Pittsburgh.
A central component of our installation was an agar topology of Pittsburgh. We placed Physarum Polycephalum mold spores on this topology in the centroid locations of "tech culture", and used a robotic plotter system to feed the mold with Soylent in a pattern defined by data marking expansion of tech/startup companies.The resultant mold growth pattern was then juxtaposed against the original data through projection mapping onto the agar substrate.
We made multiple iterations of the agar map to get a nice uniform cover of agar with a solid translucent quality that would receive projections well from underneath. Here's our final method.
Step 1: Prepare an Image of Your Topography
Pick a latitude and longitude region for your agar topography and crop a rectangular region. You will want to turn your image black and white so it can be read as a depth map by your modeling software of choice. Additionally, you will want to apply smoothing or curve management as you see fit to make particular features prominent. We used smoothing across the whole topography and deliberately increased the darkness of the river region to make it a more pronounced dip. The above image show a map of Pittsburgh along with our cropped and modified region
Lastly, because the object we will be CNCing is actually the mold negative, the image needs to be reflected over the Y-axis. This can be done at this step, or the next step once the data is in 3d-form.
Step 2: Turn Your Topography Into a 3d Model
We used Rhinoceros to create the 3d model for our mold negative. This is specific to the desired dimensions of the negative. Our negative was around 1.5 inches thick, so we created the model with enough space around each edge.
Its important to note that creating a beveled interior edge makes releasing the mold substantially easier. Obviously the amount of beveling will depend on the local topography at the edges and the thickness of the negative, but we aimed to optimize this surface.
Step 3: CNC Mill the Topography
CNC your model at the desired scale out of the desired vacuum form negative material. Our negative consisted of 2 0.75" sheets of MDF glued together and left in a vice overnight. We did multiple CNC runs to figure out the optimal bit and bit path to use so as to create the smoothest possible negative. One of our first attempts is on the right, and our final attempt is on the left.
Holes should also be drilled in your vacuum negative, which assists the vacuum forming process. This is shown in the bottom picture.
Step 4: Vacuum Form Your Topography
Next you need to create vacuum forms of your topography. Using a vacuum former and your CNC'd model, create multiple negative forms of your topography.
These vacuum forms will serve multiple purposes: they function as the true negatives for the agar, the layer on which the agar will rest, and the support structure which helps the resting layer maintain its shape as the hot agar cools and assumes its solid shape. We experimented with various thicknesses of PETG and styrene. Once you find a particular material you like, making many vacuum forms of your topography in one vacuum forming session is helpful to facilitate the next step.
Step 5: Identify 3 Usable Vacuum Forms
Out of all the vacuum forms you made in the previous step, pick two that fit in one another well and appear maximally parallel at the edges, meaning the fit is not tighter on one side than the others. I used fine grain sandpaper to improve the fit of our most optimal pair. A third vacuum form should also be set aside to be a support. When pouring hot agar into the layer on which we are casting, we don't want to deform the plastic through heat. This layer doesnt have to fit as well as the top negative, but it should still fit well enough such that it can resist deformations.
Step 6: Clean Surfaces That Will Be Touching Agar
With a course brush, remove all the bits of MDF (or whatever else) material are sticking to the two molds that will be in contact with agar. Use disinfectant wipes to go over each mold, being sure to clean any dirt stuck in the raster indentations created by the CNC routing pattern. This is especially important if you will be growing a particular species on this agar; we want to ensure it starts with sterile initial conditions.
Step 7: Prepare Hot Agar Mixture
We referred to the proportions recommended in this guide:
which recommends ~15g agar powder per 500mL of water. In the final cast that was made for our project, I included ~20g per 500mL for a slightly more opaque look. Mix the agar powder with the necessary volume of water at this concentration, and bring the water to boil while stirring to prevent any film or clump formation. What would be super ideal is a bunsen burner with a magnetic stirrer, but manual stirring works fine here too.
The ideal volume of water can be found by filling up one of your selected vacuum forms with water, and depressing the negative on top of it to see if any displaced water spills outwards or if the volume is not enough. I used this waterline test in the picture above to find out my initial volumetric guess for my topography of 400mL was way short--you can see it barely covers the deepest part of the rivers. You want a nice thick uniform layer of agar, but don't want to compromise pressure, which allows features to become more finely resolved in the agar. Finding a desired agar thickness is something that can be experimented and iterated upon.
Once the mixture has boiled for one minute, proceed to pouring the agar into your mold.
Step 8: Pour Agar Into Mold
Before you pour, make sure your selected master mold for the bottom half is resting on top of the designated support mold in a clear area, preferably on top of a towel.
Carefully pour the agar into the bottom master, making sure not to splash or pour boiling agar on yourself. Place the top half master mold onto the bottom half and press down uniformly. If you are using transparent plastic, look to ensure that the agar liquid is dispersed evenly throughout the topography as close to the edges as possible without spilling. The more pressure that can be applied without spilling, the more detail will be resolved, and this depends on the fit of the two master molds.
Step 9: Apply Pressure and Cool
Apply uniform pressure to the top half and let the agar cool to form the topography. Using a flat piece of MDF or particleboard to support weight on top of the top half allows weight to be distributed evenly across the perimeter of the mold. If there is a particularly deep valley, you might want to add a weighted object on the interior region of the top half mold to further depress it. However you should be cognizant of creating spills or evacuating liquid from other regions. This is another instance where having a transparent plastic mold is helpful.
Let the agar cool for at least 45 minutes. Err on the side of letting it cool longer, because pulling the top half away too early can cause the gel matrix which was forming to tear if it is not fully solid.
More than likely this will take a couple of attempts to get the desired coverage and thickness. Shown above are some of my iterations which I used to troubleshoot this process.
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