First, set up your figure, pose and so on in Poser and export to Bryce as an .obj file, as usual. (If you don't know how to do that, I have a tutorial that explains it.) Compose the picture and render in Bryce as you would ordinarily do, making no attempt to make the arm anything but flesh. Export the image as a Photoshop file.

Then select the figure, and hide everything else by clicking on the Solo Mode button in the Selection Palette. Everything else will disappear. Now, if you look at your figure, you will find that there is a list of letters and numbers and things. It's usually to the right, but that can vary depending on where the figure is on the screen. The top one is A, the bottom should be U. Click on U to ungroup the group. The bottom one should change to G. If it doesn't, keep clicking on U until it does. (Unless, of course, you know what is grouped, and none of the part you want to change belongs to any group.) If you are changing just part of the figure, as I am doing with the arm in this example, use the Director's View to move around until just that part is isolated against the background. Draw a marquee around that part with the selection tool to select it. If there are bits that didn't get selected, select them by either holding down the shift key and clicking on them, or holding down shift and control (on both platforms) and clicking on the correct part from the list that pops up. When you have the part that you are going to change selected, click on the A to get the Attributes dialog box. At the top is a place for the name, which should be blank since several things have been selected here. Below that is a row of checkboxes. The first one is Neutral, and should be checked. The next is Positive. Click on the Positive box to choose it and unchoose Neutral. Then click on the check mark at the bottom to close the box. Now click on the G at the bottom of the list to group the part. You will notice that the bounding box has diagonal lines at the corners. If you want to make the next step easier, you can click on A again and name the group. If you changed just part of the figure, select the whole thing and click G again to group it

What you have just done is made a Boolean Union object out of your figure. It's necessary to do this if you are blending with a transparent material, because there are a lot of joints and messy things inside Poser figures, especially clothed ones, which show up if you make the figure transparent. A Boolean Union is all one piece, not a bunch of pieces, so all the joints and things disappear, leaving a smooth and seamless piece of water (or glass, or whatever.) I've shown the difference in the examples above. (Note that this doesn't always work for heads or some conformable clothing, for some reason. You may still have to set the nostrils, inside mouth, etc. to be transparent.) The bounding box with the diagonal lines signals that there is a boolean operation here.

Now go to the flippy triangle to the right of the Family Selection icon on the Selection Palette, and click on it to get the popup menu. Go to Select Groups and select the group you have changed. Click on the M to go to the Materials Editor, and assign the material you want for your transformation. In this case, I chose clear water. (I removed the bump, though, and gave it an Ambient setting of 7 because the magic water has a bit of a glow.) Exit the Material Editor and save the file with another name, (Save As...,) in case you need to go back to the other shading later. (You can't have too many backups.)

Render it, and export the image again. At this point, you may find that the picture is no longer working; for instance, you may decide that there is not enough contrast between the arm and the water, or something like that. If you like, you can fuss with things until they are right. Then render, export the image, open the file with the other material (in this case, the flesh) that you previously saved, and copy and paste the figure into the new lighting (deleting the water figure.) As long as you haven't moved the figure at all, it will be in exactly the same place. Render again, so the two match perfectly. (See why you have to keep the first file intact?)

Assuming that it all looks good, select the changed part if it isn't already selected, and go to the left part of the Main Interface. There are a bunch of flippy triangles there. The one on the bottom lets you choose the kind of image you are making. Set it for Mask Render, and let 'er rip.

You will get a mask with just the silhouette of the object you have selected in white against a black ground. Export this one, too.

Move to Photoshop. Open the first image, and save it with a new name. Now open the second image, and arrange the two windows on your desktop so you can see both. Click on the "v" key, or select the "move" tool, and click anywhere on the second image. Hold down the shift key, and drag the mouse over the first image. The second image will appear as a layer of the first, correctly positioned just where it was in its own image. (This works for all layers of all images, by the way. It works best if the images are exactly the same size, or were once.) Open the Channels palette, and make a new layer. Open the mask render, select all, copy, and paste it into the new channels layer on your composite image. Now you have both images, with the water one on top of the flesh one, and an alpha mask ready to use.

Move to Photoshop Open the first image, and save it with a new name. Now open the second image, and arrange the two windows on your desktop so you can see both. Press the v key, or select the move tool, and click anywhere on the second image. Hold down the shift key, and drag the mouse over the first image. The second image will appear as a layer of the first, correctly positioned just where it was in its own image. (This works for all layers of all images, by the way. It works best if the images are exactly the same size, or were once.) Open the Channels Palette, and make a new layer. Open the mask render, select all, copy, and paste it into the new channels layer on your composite image. Now you have both images, with the water one on top of the flesh one, and an alpha mask ready to use.

Select the water layer by clicking its name in the Layers Palette. (Unless you have renamed it, it will be Layer 1) Now hold down option/alt and click on the mask icon at the bottom to make a new mask that is filled with black. This hides the entire layer.

Now choose a brush, flip white to the foreground, and start painting your water arm into the flesh arm. The advantage of doing this as a mask, and not by erasing the layer, is that you can go back and forth between the water and the flesh at will. Make the transition look organic by painting in streaks, leaving tatters of untouched flesh. Streaks that are flame shaped will give that elusive sense of motion, because we are used to seeing flame shaped things moving very quickly.

Check your mask by holding down option/alt and clicking on the mask thumbnail in the Layers Palette. This will let you know if you have really gotten the clear area as clear as it can be.

To get a magic glow around the whole arm (or whatever you have changed) make a new layer by clicking on the New icon at the bottom of the Layers Palette.

Then go to Select - Load Selection... and choose the Alpha Mask (or whatever you named the mask when you pasted it in.) You may have to invert it, depending on how you have set Photoshop up. What you want is marching ants around the arm and around the border of your picture. If they are just around the arm, go to the Select menu and chose Invert. Type Command/Ctrl H to hide the Marching Ants so they don't distract you.

Use the Airbrush tool with a light color to make areas of light. I have made streaks that leap from her hand to her shoulder to give the impression of the magic moving very quickly in that direction.

Change the opacity of the layer to get the effect that you want.

For the streaks of magic running up her arm, I outlined the changing parts with the Airbrush and a bright cyan, and then made three copies of that layer. The first copy I filtered with a Gaussian Blur, and cut the opacity to 78%.

For the second copy I cut the opacity to 42 using the Dissolve blending method from the popup at the top of the layer menu. This gave me single pixel dots, but they were too bright. So I made an empty layer below it, and chose Merge Down from the flippy triangle at the top right of the Layers Palette. That gave me a layer with just the pixels, but one which I could blur and change the opacity settings for, which I did. (Occasionally, when you try this, the merged layers will be set at the opacity of the Dissolved layer, and will behave as if you are using the Dissolve blending method even when you aren't. If this happens to you, use the History Palette to go back to before you merged the layers and hide all the other layers by clicking on the eyes, so that only these two layers are showing. Then select Merge Visible and the merged layer will behave correctly.)

For the third copy, I held down the Command/Ctrl key and clicked on the layer thumbnail to select just the non-transparent pixels on that layer. Then I went to the Select menu and chose Modify... Contract. I contracted the selection by 4 pixels, and filled the resulting area with white. That gave me the bright part of the glow.

Next, I painted the remaining tatters of flesh on the back side of the arm. Since it's transparent, you would be able to see these, and putting some in adds to the realism and the 3D effect. Obviously, you won't have to do this is the arm is turning to stone, or something else that is solid.

I used the same techniques as before to make magic glows for them, and pulled all the layers under the first set of magic glows so they would be in back.

Then I used some brushes I made with light effects to get the final layers of sparkles.

And that's it! Play with it until it looks the way you want it, and don't be afraid to really pile the layers up. If everything is on separate layers, you have much more flexibility, and can correct anything that seems overdone (or underdone!) easily. Don't forget to allow for transparency in the shadow or in any reflections, while you are doing all this. The transparent shadows and reflections will be there in that second layer. Just use the mask to make them appear where you need them on the picture. It's that sort of thing that will make the arm really look like it's turning to water, even when the viewer isn't aware that they are seeing that shadow at all. Most of what we do is meant to hit the viewer at a visceral level, anyway.

The same method can be used to make a model fade from any substance into any other, as you can see here, no matter what was used to render it. And you don't have to do all the post work with the magic glows and sparkles unless you think it will add to the picture. Just remember to have fun with it!