So you probably know what pipe mud is. If you don't it's pipe ash mixed with water (or saliva) to form a paste. This paste is used in a pipe chamber to fill defects and change the chamber geometry (usually in the bottom). This can be used to fill a sandpit on the inside of a chamber, raise the bottom of a bowl to the level of the airway, or plug the sides of the shank in a corncob chamber. This paste dries in a day or so and over the course of many smokes the pipe mud will absorb tars, oils, and moisture and will harden to become an indistinguishable part of the surface of your chamber. Pipe mud is a great tool and is pretty much like duct tape when it comes to doing anything in your chamber. Now, most people use pipe ash for this but you could just as easily use cigar ash or cigarette ash. They all pretty much act the same. They stick together when wet and form a workable paste. What I've found to work the best is actually wood ash such as from your fireplace. This performs the same function as pipe mud but, as I've found, more effectively. Although it is similar to tobacco ash it is also quite different. I actually call it pipe cement and I'll tell you why.A major component of wood ash is calcium carbonate. This is what makes this ash a good fertilizer, especially for plants which need a lot of calcium. Calcium carbonate is also known as limestone and happens to be the major component of... *drumroll*... portland cement. In manufacturing the limestone is crushed into a powder and heat treated at high temperatures to become chemically active. Your fireplace burns hotter and more completely (in a chemical sense) than your pipe. This creates ash which not only contains more calcium carbonate but which is also much finer and which contains calcium carbonate which has been heat treated at a higher temperature. This makes your fireplace ash chemically different than your pipe ash. To put it shortly: your fireplace ash is a cement, your pipe ash is not. Take a look at this picture:Left is pipe ash, right is fireplace ash. Notice that the fireplace ash is much whiter and has a fine grain size. Notice that the pipe ash is black and clumpy.So you're thinking: so what? Who cares if fireplace ash is a cement, pipe ash has worked to make pipe mud for generations why should I switch now? Well, you're right there is no reason to switch and pipe ash works perfectly fine as pipe mud. I'm just an engineering student that works in a cement lab so I find this interesting. And also, as I'm about to tell you, pipe cement has some up sides where pipe mud falls short.First of all, pipe cement has much more agreeable working properties. That is to say it handles nicer as a paste. Since pipe cement is a fine powder you can slowly mix water into it with your pipe spoon until it becomes the consistency of play-dough. Pipe mud on the other hand tends to be a crumbly mess like dirt or... well... like mud. Take a look at the following photo:Notice how the cement has been made into an even mass while the pipe mud is an uneven and crumbly. At this point you can simply put the ball of cement on the end of your poker tool and lower it into your chamber, then you can press it and form it into the perfect contours in your chamber. Pipe mud does not handle half as nice as pipe cementNext there is the drying time. Pipe mud dries, meaning the moisture from it needs to evaporate before you can smoke your pipe. This takes 24 hours or so. After that the pipe mud is delicate until it has been smoked enough to absorb tars and oils and become hardened like cake. Since pipe cement is a cement it cures rather than drying. By this I mean that it reacts chemically with the water and the H2O molecules become part of the cement. This takes about 4 hours and you have a smokable pipe. After 24 hours you have a fully cured cement which makes a satisfying "clink clink" sound when you tap it with your tool.Your pipe cement will also cure to be much harder than the pipe mud. Take a look at the following photo: I pressed with the same amount of force with my spoon (3 hours after initial mixing) and you can see the difference in strength.So now you're thinking: so what? This is a pipe not a bridge why should I care how strong my pipe mud is? And you're right it's not that important but if you drop your pipe and your lump of pipe mud falls out that would not be great.There is also the matter of porosity. Pipe cement is amazingly porous! I estimate it can absorb at least its own weight in water. And dries fast. It is definitley superior to pipe mud in these terms. It's actually so great at picking up moisture and oils and tars that I've even thought about lining a briar pipe's chamber completely in pipe cement (kind of like meerschaum lined briar pipes).So there you have it... there's my little blurb about pipe mud and pipe cement. I don't claim I "invented" this or anything. I'm sure this has been thought of before but I have not found it in my search on this forum so I thought I should write this down here. If anyone does this I'd love to hear your personal experience with it. And if there is a reason people don't do this I'd love to hear it too! Although, from my personal experience I have not found any negative side effects of pipe cement.