Whether you are conditioning old slip or recycling slip trims and dried slip, it is important to make sure you condition it to its appropriate balance of gravity and viscosity. Even as you work with ‘new’ slip, you should run periodic checks to see if your slip is in balance because overtime even with daily use, evaporation and other factors can cause it to become ‘off’ balance.

Gravity is the ratio of water and clay. Viscosity is its fluidity. Knowing both of these factors when you work with your slip will help you determine if you need to add clay or water or chemicals (i.e., sodium silicate or Darvan) to create the optimum slip. Ideal gravity is 1.75 to maximum of 1.8

It is important that you have your slip in balance for several reasons. If you have too much water, your ware will shrink too much and your molds will wear out faster. Having too much clay will cause your ware to not shrink enough to release from the mold and can create stress cracks and air pockets. To have the chemicals out of balance, your ware can become more brittle and increase hard spots. These are just some of the more common issues of improper slip.

So how do you know whether you should add clay or water or chemicals? Basically it is rather simple.

If your gravity is too heavy add water, if your gravity is too light add clay. Once you have your gravity in balance, then you need to check the fluidity (viscosity). If the gravity is correct but the slip seems thick, you need to add a defloculant (sodium silicate or Darvan for example) to make it more fluid/thinner. These chemicals only require small amounts. Mix the defloculant with a small amount of water so it will blend into the mixture, do not add it directly into the slip without diluting it. Depending on the size of your batch of slip, this can be as little as a few drops. If you add too much, your ware will ‘look’ different, almost leathery and sometimes discolored. It will just ‘feel’ different. It will set up faster, but it will be more fragile, brittle and could even crumble. Oftentimes, it is even harder to clean. There could also be an increase in hairline cracks that may not show until after firing. If you overdose your slip you will need to add clay and water to bring it back into balance. This can be a real ‘pain’. If you do not, the bisqueware will not be as strong, less durable and subject to easier breakage.

If your gravity is correct, but your slip is too thin, you can add a small amount of sodum bicarbonate or epsom salt (diluted) to thicken it (floculate). Warning, keep checking gravity to keep it in balance.

One other thing of importance. When you have too much water in your slip, it will soak into the molds making them too wet to work with. When over saturated, the wear on the molds will decrease their life and use, quickly wearing out the detail. Molds are expensive, so you want to get as many castings as you can and balanced slip will ensure maximum productivity.

A gallon of good ceramic slip should weigh 14.5 pounds (plus a smidge). And it should have a gravity ratio of 1.75. A simple way to check your gravity is to take a container that will hold one or two cups of slip. Place the empty container on a scale and calibrate the scale to indicate zero. Then add one cup of slip and check the weight. A gallon of slip should weigh about 14.5 pounds (plus a smidge). Divide that by 16 cups and a cup should weigh about .91 pounds.

Take a container that holds one cup of slip. Weigh the empty container and calibrate your scale to equal zero with the empty container on it. Then add one cup of slip and weigh it. From there you can tell if your slip is too heavy or to light... There are some good videos on youtube on making a viscosity cup from a styrofoam cup. I suggest when you first test for viscosity that you do it using a NEW box of slip to find the rate the mfr uses. Keep records. It should be the same each time.