PeterH

First a few caveats.
1) As you ask what is the diameter of the wire I assume that you haven't got it yet. If so, why use expensive nichrome when you are operating it at such a low temperature? Oxidation of the conductor isn't going to be an issue, so nichrome's big selling point doesn't apply.
2)  At the temperature you mention the wire looses heat  mainly through conduction and convection rather than radiation. These are very dependent on the (changing?) surroundings of the wire. 
3) The amount of heat required to melt snow off the wall (I'm assuming the fabric is inside the wall) may significantly exceed that required when the outside conditions are dry. Do you have any guestimate of the weight of water that might need to be unfrozen? (It takes about 0.1 Kwh to melt 1Kg of ice once it's at 0C.)

4) While it still needs careful engineering, I suspect that a thermostat-based solution would handle the wide range of conditions better.
Then a reference from the Electrical Engineering section of a Q&A site
How do i find nichrome temperature
https://electronics.stackexchange.com/questions/84516/how-do-i-find-nichrome-temperature
... I rather go with the second answer
You can't. Sorry.
The temperature of the wire depends not only on the electrical power on this wire, but also on the ambient conditions - i.e. how the heat is dissipated. This way, the temperature of the wire will not be constant in time, but will change depending on the air temperature and movement, materials that are in contact with the wire and so on.
In order to make the temperature constant you will need to make the ambient conditions pretty stable, or to use temperature regulator that to regulate the electrical power on the wire in order to get the same temperature in different conditions.
As long as the theory behind such phenomenons is too complex, the calculations are actually possible, but very complex. In most cases, experimental way is much more reliable and useful.
Simply use regulated power supply and change the wire current until the temperature becomes what you need. But always remember that in other conditions it will be different.
PS I see that StackExchange have tags that direct heating questions to interested readers. Maybe posting there would give you a better answer.
https://diy.stackexchange.com/questions/tagged/heating
https://diy.stackexchange.com/tags/heating/info
I've almost no personal experience with StackExchange, other than it's software section which is always informative.

Could you post a picture of the discolouration? Sounds like the clay is fluxing to the alumina? Are you firing over cone 6? 
Probably. There is kiln wash and then there is the good kind of kiln wash. Link here discussing various ones. I prefer the high alumina content one of 50 alumina hydrate, 25 epk, 25 calcined epk.

Woohoo! Refreshing update. Thank you! You know you’re lucky to live so close to the clay mine, right? Wish I could pay someone to to dig my clay up for me.
Something about borax, in addition to challenges it makes with glaze formulation and application, consider that it will flux your clay body (it soaks in). It can change the fit of glazes, perhaps in an inconsistent, unpredictable way. I’m talking about having half a kiln load shatter on cooling, or having a mug crack clean in half when filled with hot water. Not conjecture, I know someone personally who did work with borax washes and these things happen. There is something known as calcined borax available that’s less soluble (apologies if it’s been addressed earlier in the thread), relatively less expensive than frits, and still has a sort of “purity of material” I sense you value, which could be worth exploring. 
Just saying stay loose and be ready. You’re doing great work, I love it! 

https://lindabloomfield.co.uk/  ?

I said, quick, lol!

If you can manage 5 kg of clay, I’d guess you’re at a point where you can you can attempt to replicate or make your own version of other pots. Try looking at some historical examples for inspiration, and see how you can modernize or put your personal spin on it.  Holland has some notable history with tulipieres, so that could be a starting point. If that doesn’t appeal, lots of places have made assorted Meipyong or orb shaped jar variations that would make excellent canvasses for writing. The vases supplied by a florist usually are ones that are designed around more functional considerations than aesthetic ones, but small changes to proportions can make for a more interesting piece.  You mentioned the possibility of a fruit bowl shape: a simple form like that with Swifty lyrics written in a spiral would be a very striking piece, and fairly straightforward to execute.
Once you have your form chosen, there are lot of ways to possibly transfer your chosen lyrics onto the piece: if your handwriting is good, freehand it in pencil first and then go over it in glaze pencil or painted oxide. If you have my penmanship, you could project an image onto the side of the pot to trace, you could use a number of transfer methods using photocopies or printed materials to create a template you can go over. If you have simple printmaking materials available, you can create mono prints with underglazes or slips.
Looking at your chosen clay body, you will want to fire it to the top recommended temperature to make sure the porosity of the piece is as low as possible. Mayco says your chosen clear can be used up to cone 10/1280*C, so you’re good there. The clay body does fire a bit beige/cream at that point though, so you may want to consider a white underglaze layer to pop any black writing. 

https://lindabloomfield.co.uk/  ?

If you are interested in glaze-over-glaze effects ...
Basic "evening-class stuff", but a big test tile with a stripe of each of N glazes one way, overlain with a stripe of each glaze at right angles shows you the effects of all N glazes under/over all the others. I wouldn't try with N=60 though!
...  a blank line in one direction [so an Nx(N-1) grid] shows single and double coverage of each glaze as well.
The chief drawback is that you are only showing a spot of glaze fired on a smooth horizontal surface. Which probably isn't a big issue with tests for mosaic tiles. Still doesn't explore variations of glaze thickness (although the N+1 idea helps).
PS Cannot find a picture at the moment, so I'll try to draw it. Underlined letters written on the test-tile. x/y indicates glaze x over glaze y.
            top
        A    B     C
--    a       b      c
A  a/a  a/b  a/c
B b/a  b/b  b/c
C  c/a  c/b  c/c
... here is an example with paints rather than glazes
https://www.chrishilbig.com/review-golden-open-acrylic-paints/

NASA seem to have had similar problems creating wind-tunnel models, there might be some interesting thoughts in:
Improved ceramic slip casting technique https://ntrs.nasa.gov/citations/19940015904
... and the associated patent
Not least the idea of using a shell mould (i.e. constant wall thickness).
PS They seem to apply (paint?) the shell on a wax positive.

If you prefer to cast it then a previous reference might be relevant
3D Printing a Mold for a Slipcasting Mold
https://www.instructables.com/3D-Printing-a-Mold-for-a-Mold/

May I suggest that you make a few more birds, a little practice/experimentation goes a long way.
Then maybe try for a hollow bird
How to make a hollow bird form using thumb pots
... Make a small through hole with a skewer to let any steam generated during firing escape.
You can make some bowl-ish items by draping thin class clay slabs over a balloon.
e.g. clay over a balloon https://www.pinterest.co.uk/jazzdd76/clay-over-a-balloon/
... which also mentions a few projects, such as

More balloon ideas at Pottery Ballon Technique https://www.pinterest.ca/black3200/pottery-ballon-technique/

I'm unsure that you can produce a plaster slip-casting mould with a lost-wax process. Wouldn't melting the wax contaminate the plaster surface?
A highly speculative, but possibly relevant thread, relying on the thermal degradation of the plaster.
@tkw954 Did you have any success?
PS More details on the size and shape of your master would be helpful.
 

Knowing that, I would suggest a carbon fiber heating element or flexible element often used in heated clothing. 80 meters of Nichrome is a very long distance so not sure what area you plan to heat or how you intend to distribute the heat from a single wire. My first instinct is to enclose or protect from snow, maybe move energy with air, but that may not fit your use. Maybe a full description here will sparks some ideas from others.
Just to add a bit to this we are talking about raising some area from -15c to above 0c with only still air (undisturbed layer of air near the fabric) as an insulator. The slightest breeze makes that layer go away. This can become a very large loss of heating in a hurry. Maybe insurmountable wattage actually, when we look at available batteries and storage.
I think the more accurately you can describe here it likely will improve your chances of suggestions by others.
@Ahmad.khd Just looking at this further, car mirror heaters might give us an idea of the energy required. If my quick math is right - temperature dependent - it looks like on the order of 1-2 watts per square centimeter. Likely not trivial depending on how large an area you are trying to defrost. Cold climate Heat pump technology could produce this at 1/2 to 1/4 the electrical energy vs resistance heat. 
Assuming the above is correct my thoughts go to dark minimal awning, maybe using the existing sun as practical and a ducted heat pump only when essential. Maybe even automotive heat pump.
 

Wrt @neilestrick point, you can use a slate sealer or other product to seal the surface which contacts  the wall .
If seepage significant on other surfaces, i.e. dripping moisture to a floor then may have to seal inside of pots as well.
Damp clay pots can harbour molds, some folk may be sensitive to this. Sealing the inside should prevent this.

Related thread, especially
 

If you are interested in glaze-over-glaze effects ...
Basic "evening-class stuff", but a big test tile with a stripe of each of N glazes one way, overlain with a stripe of each glaze at right angles shows you the effects of all N glazes under/over all the others. I wouldn't try with N=60 though!
...  a blank line in one direction [so an Nx(N-1) grid] shows single and double coverage of each glaze as well.
The chief drawback is that you are only showing a spot of glaze fired on a smooth horizontal surface. Which probably isn't a big issue with tests for mosaic tiles. Still doesn't explore variations of glaze thickness (although the N+1 idea helps).
PS Cannot find a picture at the moment, so I'll try to draw it. Underlined letters written on the test-tile. x/y indicates glaze x over glaze y.
            top
        A    B     C
--    a       b      c
A  a/a  a/b  a/c
B b/a  b/b  b/c
C  c/a  c/b  c/c
... here is an example with paints rather than glazes
https://www.chrishilbig.com/review-golden-open-acrylic-paints/

A visual reminder that a little water expands into a lot of steam, rather quickly.
 
 

If you are interested in glaze-over-glaze effects ...
Basic "evening-class stuff", but a big test tile with a stripe of each of N glazes one way, overlain with a stripe of each glaze at right angles shows you the effects of all N glazes under/over all the others. I wouldn't try with N=60 though!
...  a blank line in one direction [so an Nx(N-1) grid] shows single and double coverage of each glaze as well.
The chief drawback is that you are only showing a spot of glaze fired on a smooth horizontal surface. Which probably isn't a big issue with tests for mosaic tiles. Still doesn't explore variations of glaze thickness (although the N+1 idea helps).
PS Cannot find a picture at the moment, so I'll try to draw it. Underlined letters written on the test-tile. x/y indicates glaze x over glaze y.
            top
        A    B     C
--    a       b      c
A  a/a  a/b  a/c
B b/a  b/b  b/c
C  c/a  c/b  c/c
... here is an example with paints rather than glazes
https://www.chrishilbig.com/review-golden-open-acrylic-paints/

If you are interested in glaze-over-glaze effects ...
Basic "evening-class stuff", but a big test tile with a stripe of each of N glazes one way, overlain with a stripe of each glaze at right angles shows you the effects of all N glazes under/over all the others. I wouldn't try with N=60 though!
...  a blank line in one direction [so an Nx(N-1) grid] shows single and double coverage of each glaze as well.
The chief drawback is that you are only showing a spot of glaze fired on a smooth horizontal surface. Which probably isn't a big issue with tests for mosaic tiles. Still doesn't explore variations of glaze thickness (although the N+1 idea helps).
PS Cannot find a picture at the moment, so I'll try to draw it. Underlined letters written on the test-tile. x/y indicates glaze x over glaze y.
            top
        A    B     C
--    a       b      c
A  a/a  a/b  a/c
B b/a  b/b  b/c
C  c/a  c/b  c/c
... here is an example with paints rather than glazes
https://www.chrishilbig.com/review-golden-open-acrylic-paints/

A seemingly simple request suddenly becomes a daunting ponderous problem. “I just want a clear base glaze I can use to start from!” I’ve been there, so first let me say, it gets better (not sure about easier, but definitely better). Mine, 20 years ago, was “How do you make floating blue?” Thus began the Odyssey.
Here are my assumptions: You want to make pottery that people can use. You want to mix your own glazes, that you can adjust and color, and experiment with. One reason is so they’re your own, another is it may be less expensive, and finally, it’ll be much more fun than buying a jug of stuff from down the street.
To start with, @Min asked what boron frits are available to you. That’s a question you’ll have to work out. Glazes in your firing range need boron and the way we potters get it is from frits. I’m afraid I have no sense of the intricacies of of ordering ceramic supplies in the UK, but you’ll be looking for only a few specific frits. Ferro frits are popular and available in the US, many glaze recipes use them,  but there are also tables of equivalent frits from other sources. “Fusion” is another brand name of frit. The term “calcium borate frit” in a recipe is just vague enough to cause trouble. Ferro frits 2134, 3124, and 3110 appear often in cone 6 glaze recipes and are known quantities of each element in them. .
In my case, there is no store (in Alaska) I can visit and grab a bag of frit 3124. I have to order it, so I do.
There are some recipes to be found onTony Hansen’s Digitalfire website, which can be counted on, but you’ll always learn more than you planned. Stick this in your search engine: “A clear cone  6 glaze base digitalfire”
If you had unlimited selection of materials, then people would just flood you with recipes, which be it’s own problem.
So, you gotta know just what you can get before people can suggest useful recipes for you. 

As for decoration, have you ever thrown a cylinder, then decorated it by either stamping, or incising then shaping the cylinder into the shape you want?  I find this a very satisfying way to work, and if you have letter stamps, or a decent script hand you can work wonders with phrases in the clay. Then after bisquefire before glazing you can use a stain/underglaze to work color into the script and wash off the high areas. This will allow the decoration to show up under glaze layers if the glaze layers are thin enough and not too dark in color. Check out some of the posts on my blog site. . . listed in my profile.
 
best,
Pres

You didn't mention how much time you have to pull this off. You will need several tests and trials, from start to finish, before you are likely get a result that is what you intended and (or) that you really like.  So, I'd want to be sure I had enough time to deal with the inevitable process of getting from point A to point B, including set-backs. Time is especially important if it should turn out that  there is a need to switch gears and design something else. Just something to think about--how much time is available to explore & produce to your satisfaction. Just FYI, Sanbau Studio offers underglaze transfer papers (lettering sets) for ^04 to 10...maybe someone else here has some experience with a transfer method...tho it would probably not be something to use for a lot of text.

This has been a learning experience.  I've posted fresh ideas, new readers start there.

PS Some of my previous posted ideas  left here for completeness.
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A quick google failed to find a public domain manual. Although there was a site offering/purporting to have it, I'm always a bit suspicious of those.
I assume that you've checked the fuse in the plug. It looks like it might have what we call an ELT (earth leakage trip) in the UK.
from https://tinyurl.com/39sknyej

Have you tried pressing what looks like a rectangular button on at the top RHS of the blue plate (while the power is off) it might reset the ELT.

PS The "Leakage Protection Device" arrow seems to be pointing to a lamp, rather than the second button ELTs usually have.  Can you confirm?
... found a better image (possibly from a different wheel)
from https://m.media-amazon.com/images/I/81Hi6FfkgqL._AC_SL1500_.jpg

Which looks like the rectangular button is a "test" button (trips the device). So how do you reset, is it just switching it on (which you are already doing)?
So maybe I'm just saying make sure you switch it off then on again while the power is connected -and report what the lights do.
PPS If you want to risk it (they might be clickbait)
https://lb.manuals.plus/vevor/gcjx-series-pottery-wheel-machine-manual#axzz88NpegZVN
https://device.report/manual/5720904
... the second has some general advise on the page

1. Water hull and stretched membrane were both theorems introduced by F.H. Norton, Phd. The water hull in simple terms meaning a single molecule of water is nearly equal to sub-micron clay particles, and a single calcium particle. Without paragraphs of science; the theorem states that sub-micron clay particles, and calcium particles are more effective at creating plasticity because they are of equal size. Sodium and magnesium are much larger; and the water molecule has to “stretch” ( stretched membrane) to encapsulate either. That gelantious effect created when you put bentonite in clay or glaze is in part a result of “stretching” the water hull. To further illustrate is Darvan. Powerful suspension agent, but few know that Darvan works in part by neutralizing sodium and magnesium ions. This allows remaining calcium to work more potently, in addition to the strong negative ionic charge created by high alkalinity. In discussing wollanite: then you get into alternate silica structures. Polymorphs I believe is the correct term. Pure silica has a high COE 12-14, while melted silica has a low COE 4.50 or so. So silica that has undergone thermal changes due to volcanic, or heat/pressure form a different crystalline lattice- thereby lowering it natural COE. For that reason, wollastonite reduces shrinkage, and adds to plasticity due to the calcium content.  Personally, I add 2% whiting. 
2. Plasticity develops over a 5-7day period unless it is ran through a de-airing pugger. Plasticity levels climb over the next several weeks. Plasticity is never an immediate reaction. A big mistake potters make is increasing ball clay/plasticizers until it is plastic upon mixing. However, in 5-7 days they have a bag of silly putty. Your projected plasticity in a week or so govens ball clay additions; not what it feels like when you mix it. Also note that not all ball clays impart the same level of plasticity.  Kentucky 5 for example is on the lower end of plasticity; yet it is still deemed plastic. OM4 is a medium plasticity ball clay, and FHC (Foundry Hills Creme) is high plastic. You have to calculate those levels when adapting a clay recipe. A common clay recipe might call for 25% OM4, and if you replace it with 25% FHC: you will have silly putty. If it calls for 25% OM4, and you replace it with 25% Kentucky 5; it will mature to the short side of plasticity.  Ball clay specs will often give CEC (cation exchange values. OM4 has a CEC around 5.8 (last I looked), and FHC is above 9. (from memory). The higher the CEC climbs, the more plastic it is. A recipe calling for 25% OM4, can be replaced with 15% FHC to achieve the same plasticity level.  Formulation rule #2; the higher the plasticity, the more water the clay will absorb. That is also translated; the higher the CEC value, the more water it will absorb. You do not want a tile body to absorb or hold any more water than necessary to form: because excess water = higher shrinkage. Throwing bodies need enough water absorption to make it malleable, but absorbing higher amounts of water will cause it to slump or fold on the wheel. The exception is hand forming, moreso pieces that require several days, or multiple steps to create. In this case, the additional water absorption will delay drying, and make the clay more suitable for carving or detailing. 
3. In your earlier post, you listed the mineral composition of the clays you like. Did you notice the alumina hovering around 25%?  24-30% is common for ball clay and fire clays, although fire clays often drop below that. Kaolin typically runs around 37%. If you formulate using clay(s) with lower alumina and higher silica; then you will have higher SiAL ratios; 5:1 is normal for stoneware. Porcelain uses kaolin, which is higher in alumina, and lower in silica: so the SiAL for porcelain bodies hover around 4:1.  The additional formulation criteria also comes from these natural SiAl levels. Stoneware bodies naturally have lower alumina and higher silica: so the silica addtions are limited to 10% for that reason. Porcelain bodies (kaolin) have higher alumina and lower silica: so silica addtions run 20-25% for that same reason. 
Bonus note: Porcelian relies heavily upon glass/mullite development to create nearly zero absorption. This is the primary reason flux additions run in the 25-30% range. Stoneware bodies focus on PSD (particle size distribution) to keep absorption under 3% (functional ware).Stoneware recipes can have up to 5 different individual clays; which plays a role in particle distribution, but also in working properties, and final fired color. Stoneware being more dependent upon PSD, also means flux additions ae kept to 10-15% total. Flux in stoneware does develop some glass, and does lower absorption some. However, flux addtions serve  a more central target of preventing cristobalite formation.
Tom

Yes, this should be simple to diagnose for a kiln repair person. The 1/2 amp fuse only provides protection for the control board, its controlled loads  and the transformer itself. So check ALL low voltage wiring to make sure no cuts and scrapes are touching anything. If you Unplug the orange, blue, white that will test the control board and its outputs. Unplugging (red, black).checks the low voltage output  side of the controller (basically the relay load)
 Your transformer appears to  possibly have burn marks on its secondary wiring side ( might be the smell) could be the picture though or could be from production. This could be  from production, or defective as a result of overloading it.
Secondary voltage — The nominal voltages measured - orange - Blue = 12VAC, orange - white = 24 vac, white - blue = 12 vac. This is a 24 vac center tapped transformer.
The red and black are the output of the controller and ought to be 12vdc (nominal) when cycling the relay on. Green appears to be unused.
All of these items can be checked for amperage draw by your tech to see which if any are drawing more than 1/2 amp.
This should be your transformer specs https://www.mouser.com/datasheet/2/410/FD5_24-781304.pdf the U shaped jumper appears to  be internal from the factory on this transformer.
This should be your kiln wiring diagram https://eadn-wc04-7751283.nxedge.io/wp-content/uploads/S0743-1.pdf
I would double check all low voltage wiring for a short to see if that overloaded the secondary of the transformer. You may need a new transformer if it cooked itself a bit.
My experience paragon designs in accordance with code, UL and CSA so their wire size ought to be fine. Not sure who or where the undersized thing came from, but that saying seems a bit suspect to me.

A google for dutch molds turns up several hits, including
Catalogue at https://www.dutchmolds.nl/ ... but seems to be in Dutch.