There were two odd things I noticed right away on the arrow block: one is the very odd discoloration around the arrow itself (see Picture above), the other which is better seen on the picture below is the fine line that is seen going through the arrow. With the right camera angle the line shows up really well. These lines can be seen on quite a few blocks at Tiawanku. You can also see it on the Kallianttla page.
The discoloration around the arrow could look as if someone had taken a mold of the arrow in the last 20 or so years with newer molding materials like RTV silicone etc, leaving discoloration where the molding compound was in contact with the rock. If that were true, however, it would leave the inner base of the arrow discolored, which is not the case. It's hard to say exactly what has happened to it; all we can do is share some ideas on what may have happened.
When making fiberglass parts and having difficult corners to deal with that fiberglass will not conform to, it is common practice to mix a thickening agent into the resin to make a paste. This paste can then be pre-applied to fill difficult corners; then the fiberglass will drape over the resin fillets better, leaving the thick paste to pick up the details and corners. This can sometimes leave small signs that this was done. When I saw the arrow rock the first thing that came to mind was that this discoloration outline was the result of such pre-fill. If they did have a way to make cement or a way to soften stone they would have seen on the first application of cement into the mold that the cement does not want to fill into corners. Now a days this in not a problem, as companies that do cement cast work have tables that vibrate to set the molds on, helping the cement settle into the finest details and raise the bubbles in the cement up to the surface.
It is possible that the person preparing this mold knew this was going to be a problem from other tests and pre-filled these corners first to pick up all the detail? When making this block, he may have used up the last bit of mix that was made for another block. Then if while he was doing the pre-filling, someone else was mixing a new batch to fill the rest of the mold, this could explain the two slightly different colored mixtures. It is also possible that it was the same mixture, but putting it on at two different times caused the discoloration just like on the home page where I cast the cement part for the picture and ran out of cement and had to mix a little more to fill the rest of the mold: this too left different colors between the two mixtures.
Regarding the faint line: this looks like a parting line. It could be that this was the spot that two different pieces of wood were glued together. The glue could have left a slightly raised line that was picked up by the cement. As you can see in the picture I left the glue on the wood instead of wiping it away which I would normally do.
When I make molds for models I always put the parting lines under wings or hatches where they will not be seen so not too much additional work is needed to prepare it for finishing. I find it interesting that the arrow was placed at its widest point to cover this line, making it more esthetically pleasing.
The lighting on this picture shows the tooling line. The arrow is placed in such a way that it covers most of the mold parting line up.
On the cement part I poured on page one, (also see part below) I did notice that the cement left a faint depression where the parting line was. If resin is used on a mold like this to make a fiberglass part, the resin always seeps into the parting line leavening a slightly raised section on the part instead of a depression. I don’t know why cement does that, that will take more experimenting.
Maybe the two pieces of wood weren’t glued together allowing air to get through during the curing process and leaving the same faint line. You can also see the discoloration outline where I had to fill the rest of the mold. Also note the trapped air bubbles on the surface, these are seen on most of the blocks at Puma Punku.
This almost looks more like the line in the arrow rock than the model I made at the bottom of the page
Above you can see what happens when cement is poured into a mold and no extra care is taken to pre-fill the corners. This was fairly wet and fine cement. Notice the air bubbles that are seen in the cement that get trapped and have no way out; if a vibrating table were used this would all fill in. The area around the arrow did not fill in too badly. On the left of the arrow and on the bottom you can see that it did not fill as well.
Without pre-filling corners the detail is not very impressive. Notice all of the air bubbles on the face of the part.
Above is the balsa plug that will be used to make the arrow rock. This is the shape of the open area in the block or the negative of the cavity. The red arrows show the undercuts that cement would not want to fill. This is why you would need to take the time to fill these areas first: if not this would leave voids in the final part and would not pick up all of the details of this shape.
You have to wonder why such a difficult shape was made in the first place. Can you imagine trying to chisel and shape these fine points with tiny cooper tools? When I finally got to Puma Punku and saw this block and the serious undercut in this shape, it really supported my theory on being molded with the corners pre-filled before pouring in the rest of the block. On all of the pictures I saw before hand, none showed the complexity of this shape.
Other than the sides of the mold, this is almost ready to pour the cement over. You can see the two boards glued together and the glue slightly above the surface. I will pre-fill around the edges, then fill the rest with cement. If a mold were to be made for a huge heavy block, the balsa base would probably have to be end grain balsa so that you would not crush the wood. This means the balsa grain would be seen from the top and not from the sides. You can see what this looks like on the Kantatallitta page.
Balsa can weigh anywhere from 4 to to 24 pounds per square foot so they could pick which density is better suited for their application. They could have also used regular wood, but balsa is much easier to work with and would make more sense to use. Maybe the frame of the mold could have been of hard wood and balsa would be used for the more complicated details.
The finished part
If you go to youtube you can watch this process as I did it,
you can see when I started to pre fill the mold, oddly enough at 6 seconds in, a little bit of the cement dripped off the tongue depressor leaving almost the same marking that is on the real arrow rock. This was unintentional, and I noticed it after the balsa tooling was removed. I wanted to redo this stone and fix the imperfections but decided I would keep it due to this flaw. I should have mixed the pre-fill batch with a bit more brown coloring so it would show up better. You can see how filling these corners first really helps pick up all of the details.
The grain size of the cement I was using looks to be much finer, picking up all of the detail of the smallest flaws which would not be visible with a more coarse mixture. I did end up having a few air bubbles trapped in the part but not bad for this first attempt. As you can see I did fill the corners, have the line in it where the wood was glued together (although a bit to deep), and I have the color distortion around the arrow.