Plaster Mold Making

in this video we're going to go over how to make plaster molds specific to the ceramic process to begin you're going to want to find an object that you want to make a mold of you're going to need to identify whether that object is hollow or solid meaning if submerged into a bucket of water will it float to the top if it is hollow then you're going to want to use other materials you could use sand clay plaster rocks but use other materials to fill it so that when submerged into a liquid it will sink make sure you fill it completely and after you've filled it you want to make sure to cap it off with something you could use oil clay tape epoxy number of materials you could use to to fill it to keep those materials from getting out next we want to identify undercuts or details that are going to cause the mold to potentially be more complex or it could potentially cause the object to get stuck inside the plaster when it's hardened so you're going to use oil clay to fill those voids or those cracks or those details because oil clay unlike water clay won't shrink over time it won't crack so you can put in a lot of time and detail and not worry about that changing when you walk away next identify your seam lines oftentimes the objects you're going to use are made in a factory and so they already have existing seam lines that can act as your map that you can follow to make your mold here the light bulb doesn't have those so i've used a permanent marker to divide it in half we're next going to use water clay to bury half of the object essentially we want to bury one half so that the other half is exposed and can be completely covered with plaster that's how we'll make our first half use a ruler to measure the height of your seam lines so that it is level next you can begin using the water clay to bury the bottom half don't worry about being too detailed right now because you'll go back as you can see here with different tools to sharpen that up and get right to your seam line use a ruler and another tool to mark measurements that will identify the the width of the mold you don't want to make your mold too thin because it will either break easily but more importantly it won't be able to absorb water from the clay as it's intended to you can see where i made that line i have a lot of extra clay on the outside that's intentional i can always cut it away later it's really difficult to add it on here i'm using the back end of a screwdriver to create a key something we'll talk about later i'm also adding this little coil on and this is so that if you want to use your mold for slip casting it acts as the opening to pour the slip or liquid clay in to your mold you're going to be able to make two shapes of molds the first one i'm going to demo is cylindrical some here i'm using metal flashing this can very easily be changed to different diameters or sizes it is important that you have overlaps so you wouldn't want just a single um circle of flashing you actually want it to go around a couple times once you have the correct size or length you can use it to gently press down into the clay where it'll hold its shape here i'm identifying the fill line it's important to have a fill line because so easily you can over pour your plaster making your mold much too thick much too heavy so have a line inside where you know when and where to stop good news since making this video we have acquired a far superior material that will replace the metal flashing it's much easier to reuse it's much safer to use and to create cylindrical molds with next we're going to show how to make rectangular plaster molds you can see unlike the cylindrical i'm starting with actually cutting off the extra clay once i have identified my perimeter careful when you do this not to change the shape of the clay you're using to bury your form it is malleable so too much pressure it'll change shapes these are called cottle boards you will need four every time you make a rectangular mold here i'm demonstrating how to set them up they're very simple but can be confusing if you've never used them before you can see they're kind of an l shape we have the bottom part of the l always facing out and we're following the bottom of the l around until we meet up with the the first board see how easily i can change shape or size gently pressing them on and using hand clamps i'm only putting on three and not the fourth because it allows me to easily get my hands into those into that space and to make small coils to fill the seams where the clay meets the cottle board this is very important if you don't do this you could potentially have what we call a plaster disaster the plaster will leak through that seam and go all over the table and floor and you'll have to start from the very beginning again you also want to make sure to do the corners of the mold just a small coil will work does not need to be very thick but make sure you run your finger up and down so that it's filling that seam these are releases or oils or soaps basically these allow you to take your object out of the plaster mold after you have created it not doing this can in the end cause your object to lock into place and cause you a lot of frustration using the ventilation system is absolutely essential when you're working in the plaster room specifically when you're using the plaster in a powder form here i just refilled our container and now i'm getting ready to calculate my measurements how much plaster do i need how much water do i need all measurements should be in centimeters milliliters and grams first start by calculating both the volume of the space of your mold the interior and the object next for rectangles you use height times width times depth minus the volume of the object to get your milliliters of water and for a cylinder radius squared times pi times height minus the volume of the object to get the amount of water second calculate the amount of plaster needed there's an equation or a ratio that you need to memorize the water to plaster ratio one part water to 1.3 parts plaster great news one milliliter of water equals one gram of plaster knowing this ratio and that crossover allows you to get the exact same consistency or mix of plaster every time here i'm setting up the scale we want to always work in grams and after you've put your container that you're going to weigh in on the scale make sure to tar it or bring it to zero next put in your water we always do water first not plaster and now i'm weighing the amount of plaster based on my calculations notice i'm slowly doing this this is important allows the plaster to slowly mix with the water allow the plaster to slake in the water for three to four minutes this is a good time to check everything else make sure you have an area to clean up you've got your tools ready you'll use the corded drill to mix the plaster for three to three and a half minutes mixing too long can cause problems mixing not long enough can also cause problems with hand mixing you're going to do six to seven minutes there's less energy going into the plaster so it takes a little bit longer of mixing to get the same consistency here i've brought the plaster over to where i'll be pouring and i'm rocking it back and forth basically vibrating the the bucket so that air bubbles will float to the top and when they do come to the surface i'll use rubbing alcohol to give it a little spritz and it'll break the tension and pop all those bubbles don't rush pouring the plaster in you want it to pour in slowly and slowly work its way over the object covering all the details and forcing any air out so that you don't get air pockets stop at your fill line and you can use your finger to dip down to the high point to check and make sure you're you've got the right amount of plaster in use the back of your hand to break the surface tension while the plaster is wet basically this will make the top nice and flat and make sure that it's it's filling all the corners after you've poured dispose of any extra plaster into an old box any type of trash put in the trash can get rid of it and use the rinse water in the rinse bucket to clean your bucket clean your tools and then take it to the sink and give it a second rinse this is an example of waiting too long classrooms mixed you can see how chunky it is after it's been poured wrap your work and we're wrapping this not so much for the plaster but the clay that's underneath if we don't wrap it that clay will become dry and be rendered unusable again we'd have to recycle it and it's just a very long arduous process so wait 45 minutes and you can begin taking the coddles off here with metal flashing you do need to be very careful those edges can be sharp the clay below you can just start peeling off and put back into the bucket as long as it's not too dry these two tools a ruler a straight edge or that loop carving tool both work well to scrape the top of the mold get rid of any sharp edges make it nice and flat you're going to be turning the mold over to make the second piece so you want to make sure that that surface is flat and it's not going to be rocking the whole time you're working on it i'm pulling a wire tool carefully through so that the clay will separate from that bottom board and now with that clay peeling it all off um you'll see taking it to the rinse bucket or to some type of water dipping it in and putting it back into the bucket i took it from you do this if it seems like the clay has dried significantly while working on it if clay is too dry it makes it really difficult for the next person that wants to make a mold using that same loop tool or some other straight edge clean up that that top part of the mold careful to identify the seam line you can use a ruler like this some type of small straight edge with a 90 degree corner to get a very clean seam between your object and the mold sandpaper works well i'm here i'm cleaning up the key that i talked about with the screwdriver i'm also cleaning up that little channel now going back with water to wipe away all the crumbs and excess clay i'm using this loop tool to make another type of key i actually prefer making my keys this way a key is basically the fingerprint of the mold that makes it unique and makes it so it only fits together a certain way these are registrations so when it's time to use the mold it will always lock back into place the exact same way so you saw me make a key with the screwdriver when the clay was soft then i just made clear keys with the loop tool after i pour the first mold both ways work next you want to use murphy's oil to soap your mold you should do this multiple times to make sure that after you pour the second piece or any additional part of the mold that they will separate now creating the second half of the channel so building back in that clay once i do this i'm now ready to start putting on the metal flashing again to create another cottle get it as tight as you can again we want to avoid plaster disasters so we really want to make sure that those seams are filled don't forget to again make your pore line have a place to stop notice this time that line is much closer to the top of my cottle again the coils don't need to be very thick to be effective it's quite often people make very very thick coils much larger than needed it's really just essential that it fills that seam like a nice bead of caulk going back in with murphy's oil a second time again just to make sure that that plaster and my object are both soaked so that they will release later next you're simply going to repeat that process calculate the volume measure your water measure your plaster let the two slate together for three minutes mix the plaster pour the plaster and clean your tools take your cottles off again clean them and also make sure to scrape the top of the mold make sure it's flat so that it doesn't rock back and forth when working on it and it's going to be easiest to separate your molds if they've been allowed to sit for a day a day or more once they have like here i'm simply allowing gravity to do the work tapping it on the floor eventually it pops open another way to separate would be to use the compressed air to give little spurts that will force the mold to pop open the third way would be to use a rubber mallet obviously we want to avoid banging on the mold as much as possible but sometimes we need those extra vibrations to help to get them to separate remove your object from the mold by simply pulling at it a little bit and if it doesn't come out easy use a little compressed air to spray it and pop it out after the objects out you'll want to use a mix of 50 50 white vinegar and water this will allow you to wash out any of the release agents so that the capillary action of the plaster is doing what it's supposed to do after you've washed it go around the outside edges of the mold you can see a simple ruler or the loop tool will allow you to do this but essentially you're going to want to do all edges lightly trimmed so that you don't have pieces of your mold chipping off while working on it in the ceramic studio chips of plaster if they get into the clay can cause what we call lime blowouts or explosions finally set the mold on the shelf and leave it for three to four days if it's just sitting on the shelf or if you have it in front of a fan and then the sun one to two days should be enough now that the molds are dry we're ready to use the molds first things first we need to prepare our clay when you pull it out of the bucket it's going to have a lot of air bubbles and be uneven so we need to stack it pound it shape it so that we can homogenize it and take away any air bubbles you can see here i'm doing a technique called ram's head wedging it's a i'm keeping the palm of my hands on the side and in front and pressing down small movements so not to stretch it out and fold it in on itself that'll create air bubbles essentially i'm doing this to homogenize the clay and make it even and empty of air bubbles next we need to make slabs there's three techniques for making slabs the first technique simply using your hands to pound the clay out stretch it kind of like pizza dough you'll see you can stretch it to start to get a longer wider shape and then using the rolling pin being sure to lift it up from time to time to separate it from the plaster then the second method very similar to the first simply uses now sticks on the side to make sure that it's the same thickness all the way through it's a bit more exacting if you're looking for a very specific thickness of slab be sure to pull the plate away from the plaster before lifting it so that it doesn't stick and rip and the third final technique is really best suited for smaller casts smaller objects essentially making a large square of clay or rectangle of clay and using those same slab sticks on the side and a wire tool i can slice through the clay and create several slabs very quickly you can see i'm making sure that the slab sticks are against my hips so as i pull through they don't move too much once you have your slabs made to put the slab actually into the mold you'll want to cut a piece larger than the actual size of the object because you're going to be forcing it down into that cavity kind of like a pie crust it's okay to have it fold over itself it's going to be very rough i'm adding a coil here to create more structure in the sharp corner of the light bulb in the bottom of the mold once it's all pressed in you can use a number of wood tools to cut the excess clay away be sure to use wood because again we want to avoid any chips of plaster getting into the clay metal tools will definitely create those notice how i'm pulling the clay away from the opening so that it's not pulling the clay away from the wall that will happen if i use the wood tool and go towards the inside sometimes helps to use a rag to do a final press really get those details make sure it's pressed in our fingertips are very pointy and sometimes can thin the clay out in areas unintentionally you'll want to take a coil of clay and take it around the perimeter edge of the clay where it meets the plaster this helps to create a larger surface area for the two halves to meet and connect but also i'm slightly pressing that coil above the surface level of the plaster so that when i put the second mold on top it will definitely connect in those places oftentimes people don't push the clay up enough and the connection is bad for the second half i'm taking a different approach and simply tearing pieces of slab and pressing them in when the clay is fresh and malleable if it's new you can connect them without scoring and slipping as long as you're pressing and blending the two with your fingers or thumb as you lay them on top of each other again cutting away the excess clay being sure not to go in towards the object but either rather cutting away so it's pulling it against the wall now my two halves are ready to be scored and slipped so if you need slip you'll need to use clay from the slot bucket or the recycling bucket found in the studio you can use one of the secondary containers found in the wood shop to create some slip that you can close with a lid and come back to each day and have it be ready to use directly from the bucket um you'll you you won't have a nice consistency so it helps to add some water you want it to be a nice thick cream once the slip's ready you'll go around your mold to both sides or one side at a time either works and simply scratch the surface i am using a fork here which is metal but i'm also being very careful not to scratch into the plaster and now laying slip on top of that texture you can think of the texture you're creating when you score kind of like a zipper or velcro so you want those teeth on both sides and the larger the teeth the better the connection now you're ready to put them together they can be put standing up like this or you could lay them down and put them on top be sure to apply pressure you want it to have a really good connection and seam it's not going to pop out right away you need to wait approximately 45 minutes that will depend on how wet your mold is and how how much slip you use to connect them at first it might not come apart that means it's too wet or too much slip it should come apart fairly easily once you open it gently rock the object back and forth if you try to pull it out too fast you could hurt it or alter it gently rock it back and forth when you see that it's moving you can flip it upside down or on its side and pull your object out if you look closely you can see a difference between the two halves that side being the ripped pieces of slab versus the other side that's more smooth here is our seam line from the mold you can absolutely keep those you can also hide them by simply trimming them off before you start press molding again it does help to go back and clean the mold you can use a wood tool to scrape away any excess slip that was left behind and then go back with a sponge to wipe it down any scraps or extra clay that's become dry or unusable please put back into the clay recycle bucket this will allow us to reprocess it and use it again when you wrap your work if it's really if you don't want to hurt it you could lay down some layers of plastic so that it cushions it and then cover it up and that's it

Channel: Sam Fox Shops

Views: 200,458

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Id: ga_b7xH8QuQ

Length: 29min 50sec (1790 seconds)

Published: Tue Mar 02 2021