Yes, pulp shrinks as it dries.  Think of all the water that helps make up the volume and then evaporates.
Solutions?  Why use pulp?  If you want a nice bonded surface with strength, try laminating, alternating bond paper and kraft.  How many layers?  It depends what strength it needs and how big it is.  Normally six layers would be strong but you might want to go up to a dozen.  Don't use flour paste and let it dry at least every two layers.  If you want to model pulp on top (for contours, shapes etc.) paint your laminate with PVA first which will help resist water soaking into the under layer.
Don't worry about Spring.  Central heating is great for drying.
DavidO

Yes, and use PVA to create a really strong bond.  Also keep your pulp layer fairly thin.  If you take a lump, put it between two plastic sheets and roll it thin, you can apply it more easily, using a plastic knife or similar.
DavidO

Yeah, I always do strips first. I like to then use the pulp to get a bumpy texture. I think the issue here was that I usually start with a cardboard globe (absorbent) and not the fiberglass one that I have used this time. I think the cracks are fixed; i put some tiny strips to bridge the gaps and a thin layer of pulp. It needs more layers of pulp, but I should be able to refill any little cracks as (if!) they form. Thanks for the tips!

How dry was your pulp when the glue was added?   Less water = less shrinkage.   Do you squeeze out every last drop?   I put my pulp in a pillow case and wring out as much as I can, then press out the remainder by rolling my fists over it while they support my weight.   Do you know your pulp to binder ratio?

Maybe the problem is that the fiber glass DOESN'T expand or shrink? Maybe the cardboard globe (by the way, where in the heck do you get a cardboard globe? I've never seen one and would love to use one!) gets wet with the pulp and expands and then shrinks back right along with the pulp,when the fiberglass doesn't?

No not in the sun, it's winter and very cold! The cracks happened after it was all dry and painted. I have contemplated concrete as a material, but that cracking of which you speak would definitely happen with our extremes of temp here. It is my plan to learn to weld! Shouldn't be any issues then?

The cracking with the concrete doesn't happen as long as you use the minimum amount of water and don't set the sphere in the sun, and then keep them moisture (under plastic) for at least a couple of weeks.  They look quite gorgeous, actually!  I've done most of mine on plastic balls.

I guess the only thing you have to worry about with the welding is rust!

Sue

A premixed Type N masonry cement would probably do fine for that use.  You don't want gravel with mortar work, too lumpy.

Always use gloves with cement (thin latex is great for finish work, but not really durable), and a mask with dry cement (you can remove it when the cement is thoroughly dampened). When that stuff gets into your lungs, it stays there.  Eye protection is good, too.  Cement is pretty caustic, esp to mucous membranes.

You would have to make small batches at a time.  The bag should tell you what the set-up time is, and however much you mix needs to be used up within that time.  And no matter what some guy tells you, you CANNOT add water to it when it starts setting up, to keep it moist! NO, NO, NO!  It will weaken it and your hard work will crack.

Add half the estimated amount of water and moisten all thoroughly, then slowly add small amounts of water to reach the right consistency.  And it's dead EASY to PASS the right consistency!  Practice is the only way to learn.  Pack some in a cheap plastic cup and then unmold it immediately.  If it slumps too badly, it's got too much water.  If it seems dry and doesn't stick together well, it's too dry.

Due to your dry climate, you would have to keep misting the section you're working on so it doesn't dry out too fast.  Cement curing is a CHEMICAL CURE, not a drying cure.  It takes 28 days to cure, period.  The longer you can keep it moist, the stronger the bond will be.  Mist and cover with plastic when soft, hose it down and cover with the plastic after setting (initial hardening).

If you have to stop and start again the next day, thoroughly wet the first section before you start the second one.  Wet cement will not stick to dry cement.  Guess how I know that?

Keep your cement dry, preferably in 5-gallon plastic buckets with lids.  If it has hard lumps in it, take it to the dump or dig a hole in an out of the way spot and bury it.  It's history.

Okay, I guess we've digressed far enough from papier mache...

Sue

Laura, is there any chance that the cracking part may not have been completely dry before you painted it, even in your climate?

It just sounds... wrong.  Esp the continued cracking.  It's like a mystery where the answer is obvious, but I'm missing it.

Sue

A Google search for "Celluclay" and "shrinkage" returns over 8 thousand hits- and almost all of the first page of returns mention cracking.    Many reduce/eliminate significant shrinkage by adding a plastic medium- PVA glue or acrylic.   One such poster used 30% PVA, while another also added flooring acrylic.   Others uniformly textured the bonding surface.   If your shell bonded to the substrate in a single place, stresses wouldn't be distributed evenly.

PVA = Polyvinyl acetate; it is most likely what you referred to as "white glue", commonly used as a wood adhesive.   If you're striving for simplicity, cooked wheat glue might appeal, as it is purported to be more hardy at low temperatures.   Cooking activates the gluten, transforming the wheat substantially; it's hardiness is well documented.   Rodents are no more attracted to gluten bonded paper pulp than a piece of wood.

PVA (white glue) comes in differing qualities. Avoid the cheaper, thinner white glues sold for children's crafts as they won't be strong enough.

I buy my pva from builder's merchants. You can get a really big tub at a reasonable price and its strong. The glue sold in craft shops is so much more expensive. OK if you only want to do one small project, but not if you are becoming a papier mache addict!