One word of caution: There really isn’t a single epoxy resin out there that won’t yellow and chemically change over time. These are not archival materials. It is best if none of the glue is coming into contact with the surface of the clay work, as exposure to sunlight will cause the material to breakdown faster.
As a general rule, the faster the epoxy sets, the faster it will yellow and degrade. For this reason, I try to use glues that set as slowly as possible and are already formulated to withstand UV exposure and water. Marine grade epoxies are excellent, but sometimes difficult to work with given how long it takes for the joints to set.
The following materials are the brands/types I currently use:
Slow Bonding for Extra strength and Mixed Media
PC-11 and PC-7.
Okay, this stuff is Really Hardcore, but a little difficult to mix properly. You measure out equal parts of part A and part B by eye, and then mix the devil out of it, whipping as much air in as possible to help the catalyst and epoxy to mix as thoroughly as possible. I usually spend at *least* 10 min just mixing, and I switch the surfaces of my mixing palette mid-way through to make sure I don’t have any unmixed components on my palette or mixing stick. I then keep a small portion of the mixed epoxy around for 48 hours to make sure the glue I mixed and used in my piece set properly.
The PC-11 (cures white), is my favorite, since I can also use pigments to color the material to match the surface of my piece. I also use the colored PC-11 to level out the bottoms of my pieces to make them level and professional looking. This epoxy is also great for outdoor installation and mixed media, but as usual, it is good to test it out first before trying it on the work.
These epoxies do have a very slow setting time (12 hours!) that can often lead to the sections moving, sagging, or sliding out of position. To counter this, I always apply the thick PC-11 paste first, and then tack it into place with 5 minute gel to hold the seam together during the prolonged setting time. Even better, in order to keep the larger or more complicated sections locked together while the slow setting glues are curing, I often drill small holes in both sides of the pieces being joined, and use twisted picture hanging wire to hold the pieces together for 12-24 hours. The wire is then cut and removed, and the seam is spackled over with a slow setting kneadable epoxy/clay. A heavy-duty sealing primer is then dry sponged over the surface of the seam to lock in the chemicals and protect the glue from UV and moisture exposure.
For filling seams or cracks:
Apoxie Sculpt two part kneadable Epoxy.
This is a type of epoxy that is mixed heavily with clay, so that it moves and acts like the original water-based clay. It usually has a long setting time of 2-4 hours and can be smoothed with water and worked with the same tools you used to create the original work. I usually purchase the white so that I am able to tint it myself by kneading in mason stains and fired clay powder. This stuff is great for mimicking texture and color. Downside is that you really should use gloves to protect your skin from the chemicals during the mixing process at least. Prolonged exposure to it on your skin can definitely irritate and eventually lead to an allergic reaction over a long period of time. Using gloves to mix it, I haven’t had any problem over the last 10 years that I have been using it.
I order it from the company that makes it in order to make sure it is as fresh as possible (it does go bad over time, even unopened) from Aves:
Note about coloring Apoxie Sculpt from their website:
Absolutely…always mix A+B first wearing disposable gloves and then add the pigment of your choice. Different pigments will give different results… examples of commonly used pigments; We find that Acrylic paints give the product a gum like consistency – which some prefer, but others dislike….it does take quite a bit of acrylic paint to create a rich tone. Oil paints work exceptionally well and are the preferred colorant method – it take a VERY tiny amount to color a whole lot of product (toothpicks are helpful in swiping color onto the clay)…a little oil paint goes a LONG ways, if you mix too much paint it product will become very gooey. Powder Pigments work well – but they are messy. They give our product a bit more firm feel to it.
There are various other forms of kneadable epoxies out there, but I feel Apoxie Sculpt line has the best versatility and working time for the type of applications I’m using in reassembling my work after the firing. They have an excellent website with lots of good FAQ. They have a whole line of clay epoxies that work really well, but by far the best one I have tested is one called “Fixit-Sculpt” with a 4 hour setting time.
Important Note: All epoxies have a limited shelf life, even unopened! Finding fresh product is a bit hard to do unless you order it directly from the company (who knows how long it has been sitting on the store shelf?), but, once you have opened it, you only have about a month after it is exposed to air before it starts to degrade. However, you can extend the life of the material by storing the part A and part B in the freezer! I usually feel safe using frozen epoxy that I have labeled with an opening date up to 6 months old. Simply remove from the freezer about an hour before you want to you want to use it, and you are good to go.